I was trying to figure out how to personalize my avatar but could not find any instructions.Don't know if I missed someting obvious but tried looking at my profile & account info, etc without success.
The avatar isn't actually an EYB feature. You add it using an avatar site (I used Gravatar.com) and then wherever you post on sites that have your email address registered, that avatar will appear. So just be aware that you won't just be adding it for EYB, it will appear everywhere that you use the same email address.
That's interesting that it shows up on other posts but not on EYB. Can you let us know which avatar site you have it registered with and we will investigate further?
What if I want to attach an avatar just for this site - from the Gallery or Google Photos - to use only on this site and not on the entire Web?
Unique username - unique avatar.
It's not possible at present but I have added that feature request to our developers to-do list.