Photos are sorted in the Photos tab according to "Date/time taken" (when available).
In albums you can sort by :
"Oldest first" for example to tell a story, for events, holiday photos etc
"Newest first" for example for sharing family photos: they see the newly added first
"Recently added" (most recently added on top), for example useful to remove wrongly added photos
Dragging photos in the order you want. This order gets however lost when an album is downloaded or someone adds the photos to a new album in another account.
There are mainly two reasons to change the order of photos in an album:
Tell a story in an album.
Correct for errors, for example when photos from different cameras or camera types are merged. An additional problem is that Google Photos takes the Time zone into account for sorting, and even photos taken in the same place get a different time zone, for example because some camera's have no GPS (no time zone), photos are uploaded from different zones (during and after holidays), or got another time zone in another program like Picasa.
I still use to drag photos in the order I want in Picasa on a computer (having a backup on a computer and at least one more place is a good idea), and rename them in batch with Name-001, Name-002 etc (see further). But Google Photos does not support sorting by file name in an album. That would be a minor change with a big advantage for users wanting their photos in the correct or preferred order.
The actual options are:
Manual dragging photos in an album, but the order is lost when an album is downloaded, or photos saved (added to an album) in another account.
Change date/time in Google Photos, but the order gets still lost when photos are downloaded.
Change date/time of a photo before or after uploading, but that may turn out to be insufficient since Google Photos takes the (often wrong) time zone into account for sorting. This is also not desirable because the original "date/time taken" is no longer seen.
Although Google Photos does NOT support sorting by file name, changing the file name as shown below preserves the order when someone downloads the album. There are many tools to rename multiple photos in batch, but you need to be able to get them first in the order you want, for example by drag & drop.
You can use for example:
Picasa rename option (F2)
Remark: Picasa is no longer supported, but can still be downloaded and installed. Recommended when you still use it or even used it in the past and reinstall. See here.
In Picasa you can
Drag & drop one or more selected photos to another position.
Select all photos (or none, which also selects all). You can also select subgroups, but then you will have to carefully choose names that preserve the correct order.
Click F2 or File > Rename
Enter a name followed by -001. All photos will be renamed NAME-001 NAME-002 etc.
When you add more photos to an album, for example where the last photo is NAME-050, you can enter NAME-051 for new photos.
When you want to insert new photos between others, you can drag them in place, then rename ALL photos with a name which is NOT the same as before. I often do it in two steps: first by adding --001, then again by -001.
Picasa Export function
When exporting photos there is an option to "add numbers to file name to preserve order".
It adds 01- 02- or -001 -002 (if >99) at the beginning of the actual filename.
Works fine for finished albums, but not for albums where photos are added later. I prefer the F2 method.
Other programs which can be used to rename multiple files (there are probably more)
Faststone Image Viewer can drag & drop followed by rename.
Irfanview (PC only):
Open one photo in a folder
Click T to see the thumbnails of all photos in the folder
Drag the photos in the order you want
Select all photos (Ctrl+A)
Type B to start Batch dialog for the selected files, or directly hit F2 > Batch rename
That would be all you need to do, if the Google Photos team would implement sorting by file name. For mobile phone users, adding a renaming tool in the Photos app(to be used after drag & drop) would make it even better.
To change date/time you can for example use :
Select one or more photos
Click Tools > Adjust Date and Time
Fill in a new photo date
Choose "Adjust all photos dates by the amount" or "Set all photos to the same date and time"
Click OK
Warning:
Changing date/time in Picasa seems to set the time zone at GMT+00 when the time zone is missing, which may again lead to incorrect sorting in Google Photos. You can try to avoid this by adding the same amount (for example 1 second or 1 minute) to ALL photos in an album, but it may still not work correctly when photos from different devices are merged.
When photos from different sources are added to the same album, or when you uploaded part of your holiday photos in another zone, and the rest at home, you may still have timezone problems, since Google Photos sorts taking into account the time zone. You may have to correct the time zone and time again in Google Photos (in the web app only) as follows:
Select one or more photos with the wrong time zone, and click 3-dots > Edit date and time.
Note the actual date/time of the photo, for use under 3.
Correct the time zone for the photos in Google Photos.
When changing the time zone it automatically changes the time so that the order remains unchanged! You thus have to give the selected photos again the original time.
This user uploaded via VPN to get the time zone correct: https://support.google.com/photos/thread/240531433/google-photos-wrong-timezone
Irfanview (only available for Windows)
Can be used for giving all selected photos the same date/time, adjust the time of all selected photos with a given amount, or add n seconds to each subsequent photo.
Warning: does not work when there are not already EXIF data. For example for scanned or downloaded photos you can add a "date taken" in Picasa first.
Open Irfanview and hit T to show Thumbnails view and find the folder you want. The photos are sorted by name, for example as renamed in Picasa. If not you can drag selected photos to the place you want.
Select multiple thumbnails (using Ctrl+A for all or hold Ctrl and Shift keys and click as usual)
Go to File > JPG Lossless Operations < Change Exif date (date taken)
You can now:
"Set EXIF date to"
set a time to give all selected photos the same date/time
also check "Add additional n seconds per file for every subsequent (selected) file"
or "Adjust EXIF time by" ...
Tools for editing the EXIF data may allow more actions, like extracting a date from the filename (for example from photos shared via WhatsApp), and put it in the EXIF data.
ExifTool
https://www.labnol.org/software/change-date-of-pictures/28336/ https://sno.phy.queensu.ca/~phil/exiftool/faq.html#Q5
https://igouy.blogspot.com/2018/03/by-year-scanned-photos-for-google-photos.html
https://igouy.blogspot.com/2018/03/by-name-scanned-photos-for-google-photos.html
BulkFileChanger: https://www.nirsoft.net/utils/bulk_file_changer.html
https://geosetter.de/en/main-en/ to change time and/or time zone
https://www.relliksoftware.com/exifdatechanger/ (for example get date/time from file name)
Use for example https://www.bulkrenameutility.co.uk/ if you need to rename first
Lightroom: Select multiple photos, then update the Date Taken field.
Save the metadata to the files and upload to Google Photos.
Go to Google Photos "Albums": https://photos.google.com/albums and open the album you want to organize.
At the top right, click the 3-dot menu and click "Edit album".
You can drag-and-drop selected pictures to another position (select by clicking or tapping on the thumbnail).
I noticed issues when trying to drag & drop on a phone, it works a lot easier on a computer.
The changed order is lost after downloading or adding to another album, for example in another Google account.
At the top right, click the 3-dot menu and click "Edit album"
You see the options "Oldest first" Newest first" "Recently added (first)".
When finished, click the check mark ("Done") in the upper left corner.
Unfortunately, "Sort by filename" is not available. Please send feedback in Google Photos to ask for that feature.
Third party extension (not tried - at your own risk): see https://github.com/hamada2029/gphotos_sort
Photos in albums can be sorted by date/time. Changing date/time may thus change the order of the photos.
Be careful: this will also change the order in the Photos tab and other albums where the photo is added.
Select one or more photos
At the top right, click the 3-dot menu and click "Edit date & time"
This only changes times in the Photos database, not in the EXIF data. Changed dates are lost after downloading, but kept when added to another album, also in another account, or using "Add to library" after sharing.
Warning: there may still be issues when photos in the album have different time zones. The time zone can be adjusted in batch in Google Photos (only in the web app):
Select all photos to be corrected
Click 3-dots > Edit date & time
Pull down the Time zone menu and correct the zone. Google will automatically change the time to keep the photos in what it thinks is the correct order. You will have to (i) remember the time as it is before changing the zone, and (ii) reset that time.