If you can't find the source/credit for some fanart, look it up on SauceNAO.
You can easily enter the URL of the image or upload it from your files. It doesn't work 100% of the time of course, but it works very well. At the very least, it should always be your first step before contributing to reposting this uncredited image to ask publicly.
Because that stolen anime fanart you've seen reposted thousands of time often comes from pixiv, which SauceNAO works with.
It should be a staple! The amount of people I see post requests to find fanart, who seem desperate and yet clearly haven't used SauceNAO, because the result is there instantly, is ridiculous. Everybody share the word, please.
It used to work with Twitter, but it looks like it no longer does.
It may not work with poisoned/shielded images and so on, re: AI scrapping protection.
Example: One of my drawings I posted on pixiv before this became a thing.
Other example: One of my drawings I shielded before posting to pixiv.
(The low similarity results mostly seem to be pink and red, so it recognised the palette at the very least.)
Yet another example of how AI scrapping has limited how we can support artists online, their ability to post art and to be found. Still, a lot of your favourite fanart has been online for many years.