There was a time when it wasn't obvious to anyone that Google Hangouts was archiving all photos sent to and fro, into albums (much like Whatsapp). The Internet generally believed that Google wouldn't be so careless as to make private messages and pictures appear just anywhere unexpectedly.
Well, we weren't exactly wrong, but when you try to insert a photo in Gmail with this here…
…a dialog appears with photos from every Google-related source: Google Photos, Google+, Hangouts, etc.
I attempted to delete all traces of these photos by going through menus in Google Hangouts, Google+, Google Photos, and Google Drive, and I still can't seem to find where the older photos are stored.
How can I delete older photos that appear in the Insert Photo dialog of Gmail?
Best Answer
Log into http://picasaweb.google.com and you should be able to find older photos that you don't find in Google Photos. Delete the pictures from there.
Source (someone else with the same problem):
I'm self-posting this question and answer despite the following thread...
...because there the question is about profile photos, which people may overlook if they're not concerned with profile photos. Also, that thread didn't appear in my Google search, so hopefully this reformulation does better with SEO.