You need to make sure the album is shared. Once that's done, the fun begins...
In your browser in Google photos, click on the image you want, so it's the main one on the screen. Right click on that image and select "copy image link". Just copying the URL from the browser bar doesn't work

The image url is the one you want to put between the image tags.
Once I've posted I usually check as an anonymous account, as if the image isn't shared it'll still show up for you, but not for anyone else.
However, the important thing is the image link url, not the browser bar one. I'm sure there's a good reason for it, but it still feels annoying. Posting from a tablet is worse, as the google photos app won't give the correct URL and I end up having to load a browser there as well
