I've used cat-themed avatars here for many years. In the early days, I used a variety, from little smileys (most popular one was Spud, a motorbike-riding potato, who wore sunglasses and a smirk on his face).
I've used needlepoint images (my current one is actually a counted cross-stitch pattern, and I take my hat off to whoever completed it, as working with gold thread is
hard), images of a favorite musician (Will Millar, of the Irish Rovers), Carl Sagan (the one I used here temporarily is my permanent avatar on the AO3 fanfiction site, where I use a different name), and there have likely been a few others I don't recall at the moment.
At some point, though, I decided to use cats, and once I find one I like, I use it for a long time.
That's on this forum. On TrekBBS, I've had only 3 different avatars over the past 17 years, and only used one of them on the beta version of the forum (when we were going to switch from vBulletin to XenForo, the tech admin made a beta version available for us to "test drive" to see if there were any bugs that needed fixing; we found some, and this gave the tech guy a chance to fix them before the actual new version went live).
I use an owl picture created by apofiss (artist who does really adorable computer art pictures of cats, owls, and birds) on a couple of gaming forums and a news site. Another gaming site has several penguins as default choices, and I used one of those before settling on my 2019 steampunk "Writer" badge from that year's NaNoWriMo competition.
TrekBBS runs several avatar competitions in various subforums. There are competitions in the different Trek subforums, in Miscellaneous (their equivalent of our Off-Topic), and in the two Entertainment subforums (one for SF/F shows, books, and movies; the other is for non-SF/F). The winner of each of these gets to host the next competition (pick the theme).
I don't enter these competitions very often anymore, but couldn't resist the current one in Miscellaneous. Since 2024 is a leap year, the person hosting decided on "leap day" as the theme. Anyone or thing jumping, leaping, etc. was okay. We're allowed 3 entries, so here are my three:
Adelie diving:
Gentoo leaping:
Rockhopper hopping:
If I should win (there aren't many entries yet, but that could change), I doubt if I'll use any of these, at least at this point. But these contests provide avatars for people to use if they want, or inspiration to make others.
There are images I'd gladly use as an avatar, either out of my own collection or others I've found. It's just that avatars do tend to become part of one's forum identity after awhile, they become recognizable, and changing them takes a kind of emotional effort I don't do much of anymore. And if starting up at a new forum that's an offshoot of an old one, or where I know some of the people already, it's just easier to keep an old avatar for recognition.