While this was easy in PicasaWeb, it's a bit more challenging in Google Photos... if you've found a better way please share them below!
The way we've found that works best is via the JSON view of all of your albums in your account.
Step 1: get your User ID
You can get the ID by going to your Google+ Page, right-click on Profile, and copy that URL. Your User ID is the long numerical string like below (the Xs):
another method is by going to https://get.google.com/albumarchive, which will then redirect to https://get.google.com/albumarchive/[User ID] (thanks +Peggy K!
Step 2: create your JSON url for your User ID
This is what the URL needs to look like; replace the XXXXXXX with your own User ID from above.
This URL will also work:
Step 3: load the JSON feed into a viewer
For example, you can use Code Beautify's JSON Viewer. In the middle there is a box to load an external feed; pop your URL in there and load it up. Once loaded, search for the title in the right side search box. Scroll down to find the highlighted entry, then open up LINK / 0 below it and get the HREF value.
Or if you prefer a needle in a haystack approach, load up the JSON url above in your Chrome browser window, then do a Ctrl-F to find the title of the album you need. Almost immediately following the title you'll see the JSON url for that album like so:
(Keepers 2015 is the album title in this case)
or you'll see URLs that look like https://picasaweb.google.com/XXXXXXX/YYYYYYYY
(where XXXXX is the User ID and YYYYYY is the Album ID)
With this information in hand you can now use your JSON feed for all sorts of nify things.