Embedding Google Photos album to HTML
I wrote this widget to embed google photos gallery into website. Here is a site to generate code for website. https://www.publicalbum.org/embedding-google-photos-albums. It is quite similar as picasa widget. Maybe it will be usefull to solve your prolem.
If you have some image files, just upload them to a public folder on your Google Drive, copy its folder ID from the address bar (e.g. 0B0Gi4v5omoZUVXhCT2kta1l0ZG8) and paste it into a form at GDrives, then choose your own alias (e.g. myimgs) and voila! You can access the images one by one using
e.g. http://gdriv.es/myimgs/myimage.jpg.
If you want to embed a whole folder on your website (in a frame), you can use one of the following URLs, replacing [folderID] with your own ID:
gdriv.es/myimgs/
docs.google.com/folder/d/[folderID]/preview?rm=minimal
drive.google.com/folderview?id=[folderID]
If you prefer to get the file list in XML or JSON, you can use YQL.
Note: You can use Google+ Photos to host ans embed your images as well.
I was just looking for a way to do this myself and found that the easiest way would be using Google Drive to store the photos instead and then embedding them with HTML as per the instructions here: https://confluence.biola.edu/display/itservices/How+to+Embed+Images+from+Google+Drive+in+a+Web+Page
It may need some styling or free source code to look like a slideshow (or copy part of the iframe code from YouTube's embed button perhaps?)