Google Photos is not a real backup (copies kept for safekeeping in case the original is lost or damaged).
Please read more at Geeks on Tour: Google Photos is NOT a "Backup" of Your Photos
There is an automatic check behind-the-scenes to determine whether the exact same photo was already uploaded to Google Photos. Any change (except file name) made to the photo or a change to it's metadata may cause the photo to be re-uploaded. See How to remove or prevent duplicates in Google Photos
The "Upload" button disappeared from https://photos.google.com and was replaced by plural options under the + icon, where you see now a section "Add photos", with several options:
Import photos opens Windows explorer on a PC
Back up folders to give Google Photos permission to backup selected folders
Google Drive opens Google Drive where you can select pictures, then click on "Upload". The pics will be seen on "Recently added" unless they were already backed up to Google Photos.
From other places
Transfer from Photo collections (Facebook iCloud)
Transfer from photography services (Pixieset, Pic-Tome, Image.Canon)
Digitise physical photos (Photomyne)
Scan photos with your phone
Unfortunately you cannot directly add them to an album using some methods, but you can when you use "Import photos" or use Drag & drop (see further).
GDD is intended to upload photos in bulk, without automatically creating albums. It uploads directly to Google Photos.
GDD is a one time upload. It does not really sync. Edits done on the computer do not sync to Google Photos, and photos subsequently deleted in Google Photos will not be uploaded a second time.
Photos/videos will upload into the Photos tab in Google Photos, sorted by camera date (if present). If you want to convert folders on your computer to albums in Google Photos, you will have to do that manually later on (see below).
GDD has an option to build a new hierarchy in Google Drive which mirrors the computer folder structure, but this will use storage in Google Drive as well as Google Photos. There will not be any connection between the two copies.
There is no option to upload directly from Drive to an album, but you can find them on top in Recently added (when not uploaded before to Google Photos). See this thread .
Tip: you can rename the photos to include the folder title. That allows to search for the content in Google Photos.
When you select Storage saver (former High Quality) in the GDD Settings, photos will be compressed and will count against your Google storage at the reduced size. Previously uploaded High Quality prior to June 1, 2021 do not use storage.
This method allows you to add photos/videos directly to albums. You keep control and can create a nice album collection. It allows to edit photos before uploading and to upload selectively.
Using "Import photos"
In the Photos or Albums tab, click +, then "Import photos".
Select a folder or photos on your computer and click "Open". When you select a folder there will probably be skipped files. I recommend to select photos.
Select one or more photos (Ctrl+select) and click "Open." The photo(s) will upload to the "Photos" tab. The photos are also shown on top in Recently added, except when the photos were already uploaded in the past.
When ready a pop-up appears inviting you to add the photos to an album. Close the pop-up if you do not want to add to an album. When some photos were already uploaded in the past, they will not upload again (duplicate prevention), will not appear on top in "Recently added" and will NOT be added to the album. Therefore it is recommended to upload directly into an album if you want them in an album (see further).
Drag & Drop
You can drag & drop multiple selected photos or even multiple folders (subfolders will not be included) in the Photos tab, which adds them to the timeline, where they are sorted by "Date taken" (when available). The photos are also shown on top in Recently added, except when the photos were already uploaded in the past.
When selecting folders there will probably be "skipped items" (unsupported files).
When ready a pop-up appears inviting you to add the photos to an album. Close the pop-up if you do not want to add to an album. When some photos were already uploaded in the past, they will not upload again (duplicate prevention), will not appear on top in "Recently added" and will NOT be added to the album. Therefore it is recommended to upload directly into an album (see below)
Import from Drive
Clicking on Drive opens Google Drive.
Open the folder from where you want to upload photos
Select one or more photos.
Click UPLOAD in the right hand lower corner.
Unlike when uploading from a computer, there is no option to add to an album. You will have to find them in "Photos" but preferably in "Recently added" where they should be on top (if not already uploaded in the past).
You can turn "Backup" ON to systematically upload photos from the DCIM folder (obligatory) and other folders (optional). Note that DCIM includes the Camera folder, but may also include other collections like Screenshots. Google Photos collects everything found under DCIM in a folder called Camera.
If you have added more than one account in Google Photos, you must select one in Backup in the settings of Google Photos. Photos will always sync with that account, also when you switched to another account for viewing. When you change the account used for uploading, also older photos will upload to the new account, not only the new ones.
"Backup" ignores the folder structure on your device: all photos are just uploaded to the Photos tab. But you can at any moment select photos and add them to a new or existing album (see lower).
You can also upload manually to the account you are viewing: select some photos, tap the 3-dots > Back up now.
As an alternative you can add photos directly to an album (see further). They will also upload to the Photos tab.
Go to https://photos.google.com or open an existing album or create a new one at https://photos.google.com/albums
Drag and drop one or more selected files or folders from your computer into the open album.
ALL photos/videos will now be added to the album, even when they were already uploaded to Google Photos.
Open an existing album at Albums
Click the "Add photos" icon
Click "Select from computer"
Browse to a folder on your computer and click "Open"
Select one or more photos (Ctrl+select) and click "Open"
The photo(s) will be added to your album and upload to the "Photos" tab (if not yet uploaded)
Remarks: If you see a message that photos were "skipped," click "View Skipped" and note the file names that were skipped, then upload them again. If there are many you can just upload the whole batch again: no duplicates will be uploaded.
You can add photos from the device to albums, also after the photos/videos have been backed up. This is especially interesting for Android where the Google Photos app also shows device folders.
Select n photos you want to add to an album
On Android devices you can select photos in "folders on device" under Library
Tap the + icon (that appears after you selected photos)
Tap Album for a new album
or tap on an existing album to add the photos
Google disabled direct upload from Picasa in March 2018. Picasa continues to be useful for local management of a photo collection with basic editing and some extras such as creating collages with much more options than in Google Photos. Emailing photos from Picasa using Gmail was also disabled. Instead you can export (select the size you want) and add as attachment to a mail.
If you edited photos in Picasa3, you must create an edited copy on the computer by using Export or Save.
The "Export" method
To upload photos from an album in Picasa, you always need to "Export" to get a folder with the photos.
Select thumbnails in the library of Picasa, for example all photos in a folder. You can either use Ctrl+A or click between the photos to select none. To select photos from different folders/albums you must add them to the tray by clicking the green "Hold" button.
Click "Export" and select the options, for example "Use original size" (for image size) and image quality, for example "Automatic" or "Maximum." You can add numbers to preserve the order or add a watermark. See this link for details.
After clicking the "Export" button, the folder containing the exports automatically opens in File Explorer (or Finder on Mac)
Go to https://photos.google.com/settings and select "High Quality (free unlimited storage)" or "Original." This setting works for future uploads only. "High Quality" is free until June 1, 2021.
You can now either use the Upload button or drag & drop the selected photos to https://photos.google.com/
You also find the recently added photos on top at https://photos.google.com/search/_tra
But you can also create a new (empty) album or open an existing one, and drag & drop directly into the open album or use "Add to album" > "Select from computer"
After you checked that the upload succeeded, you can delete the exported folder on your computer (optional)
The "Save" method:
Instead of using "Export" you can replace the original by an edited copy in File Explorer (or Finder) by "Saving" the edits. The original is filed away to a hidden folder .picasaoriginals under the actual folder, which thus doubles the storage use on the computer. See this link for details. For albums created in Picasa the "Export" method is needed.
You can now use steps 4-6 of the "Export" method to upload the photos.
Immediately after the upload, you are prompted to add the photos to an album. You also find the recently added photos on top at https://photos.google.com/search/_tra
In Picasaweb all your photos were automatically added to albums. Backup however uploads all photos only to the "Photos" tab. Re-creating the albums on Google Photos can be very laborious, but the following may help:
Rename photos on the computer, so that the file name also contains (parts of) the folder name. Bear in mind that Google Photos has no sub-albums, thus for sub-folders you may have to add elements from the hierarchically higher folder(s). I also always include at least year-month, for example 2020-08 Zoo with John and Jane-001 etc
This method will allow to search for these photos after they are for example uploaded with "Backup".
Upload new photos manually. There are several options:
After the upload you are prompted to add them to an album. You may miss photos already uploaded in the past.
You can create a new album (or open an existing one) and upload directly into it, for example by drag & drop.
You find new uploads on top on https://photos.google.com/search/_tra (sorted by upload date). You also get there in the web app via Explore > Recently added.
Get older photos from the "Photos" tab in albums
You can systematically add photos manually to an album from top to bottom in the timeline. Keep a record of where you stopped. In case of doubt, you do not have to check whether a photos is already in an album: duplicates will not be added. But that method is very laborious. I recommend the next option.
After all photos are uploaded by using "Backup" and hoping that you did not delete the local collection, you can (systematically or gradually whenever you need a particular album):
Create a new (empty) Album in Google Photos: https://photos.google.com/albums
Upload the photos/videos to the album (via upload button or by drag & drop)
The photos/videos will be added to the album, even when they are already uploaded. They will NOT upload again if not edited or metadata changed - a changed filename does not matter.
Some users "Archive" all photos added to an album (3-dots > Archive), so that they are hidden from the library and shown in https://photos.google.com/u/0/archive instead. The archived photos are still shown in the albums.
I would not recommend that method though:
After you added photos to an album, the selection is lost, and you cannot archive from within the album. You will have to select the exact same photos again which is time consuming and error prone.
Archiving is intended to remove "clutter" you do not want to see in your library or share. Photos in archive are not shared when you share your library ("Partner sharing").
Some third party software claims to add automatically photos from folders to albums. This was not tested, thus, do this at your own risk:
https://www.picbackman.com/. Only free for 5000 pics, but you could pay for one month of unlimited use, backup all your photos and folders to Google Photos, and from then on add your new photos manually.
See also https://digitalfilmmaker.net/google-photos-backup/