Google announced on February 12, 2016, that they would retire and provide an archived location for Picasa Web Albums (also called Picasaweb) so they could focus entirely on a single photo service - Google Photos.
The retirement was effective on August 1, 2016, as announced at the Picasa forum (now closed).
The archived location of Picasa Web Albums is at your Album Archive here: https://get.google.com/albumarchive
More about the Album Archive: click here
How to use the archive to view, delete, or download photos:
https://support.google.com/picasa/answer/7008270
"What's happening" info including what links will continue to work and what links will stop working: https://support.google.com/picasa/answer/6383491
Per a Google announcement, "We will be creating a new place for you to access your Picasa Web Albums data. That way, you will still be able to view, download, or delete your Picasa Web Albums, you just won’t be able to create, organize or edit albums (you would now do this in Google Photos)." This "new place" is an Archive created by Google that will allow links to these albums to still work.
Most of your photos and videos will still be available at Google Photos. Google stated, "If you have photos or videos in a Picasa Web Album today, the easiest way to still access, modify and share most of that content is to log in to Google Photos, and all your photos and videos will already be there."
Picasa Web Albums that do not appear at Google Photos are: Hangouts chat images and video call image captures, Blogger albums, Photos of You, Google Profile Photos, Scrapbook Photos, YouTube Channel Art. More details here.
Blogger photos: update by Google Employee NavinParka on February 19, 2016: https://productforums.google.com/d/msg/picasa/ufLg43ti7Hc/JzxqueUpDAAJ
"Retiring Picasa Web Albums will not affect photos uploaded to a blog through Blogger; they will will continue to display on the blog even after Picasa Web Albums is retired. You will still be able to see these photos on your blog and in a new place we will be creating for you to access your Picasa Web Albums data. Picasa Web albums shared by a link will also continue to work with a redirect."
Update effective 8/1/2016: Links that will no longer work are slideshows embedded on websites; Picasa Web Albums and photos embedded on websites; and links to photos, albums and public galleries with URLs using your username.
Using PWA, there is the ability to link to (embed) photos and get various sizes (such as miniatures) of the same image by simply changing the /sXXX/ parameter (see this page). Can this be done with Google Photos?
See: Get a direct link to a picture (specific size)
Info about Picasa Web Albums Data API: https://developers.google.com/picasa-web/ "Beginning May 1st, 2016, we’ll start rolling out changes to the Picasa Web Albums Data API and no longer support the following functionality: Flash support, Community search, Mutation operations other than uploads, All support for tags, comments, and contacts. The API will still support other functions, including reading photos, reading albums, reading photos in albums, and uploading new photos. Although these operations will continue to be supported and the protocol will remain the same, the content included in the responses and the operation behavior may change."
Uploading photos
With Picasa3, users could easily upload folders from their computers into albums using the green "Upload to Google Photos" button. Photos uploaded in this way appeared in Album Archive as well as in Google Photos Albums. Google disabled this possibility in 2018.
There are various ways to upload photos from a desktop computer to Google Photos.
Please see "Upload photos to Google Photos"
Photos uploaded to Blogger won't be affected by the changes (i.e. photos added to blog posts won't break).
What happens to the links to all of my Picasa images in my Blogger blog now that Picasa is being retired? Embedded links will not break. See forum posts and blogger forum
Blogger photos are not shown in Google Photos Albums, but exist in a hidden place where you won't have access to them. This can be seen as a good thing, because you won't accidentally delete that folder and potentially lose your photos on your Blogger blog.
Workaround to upload specific sized pictures to your Blogger Blogspot posts after Picasa Web Albums is disabled:
Find out more from the Blogger Forum: https://productforums.google.com/forum/#!forum/blogger