I have used Picasa, but this was replaced by Google Photos, as you wrote.
Picasa had the official feature of providing links for external embedding.
The good thing is, that none of the links was deleted, it is still supported, but now is no more possible to add pictures. Only download and delete is supported now.
Google Photos DO NOT support external linking. This is a official statement in the support forum, which made many users unhappy (and I think this was the real reason for Photo).
I had embedded for some time Google Photos also here (before I was aware of this issue), but it was tricky and not sure, how long this links are working. So I don't use it anymore.
Cheers,
Frank