Here are some quotes:
"I thought, well, that’s weird. Why would I need a license? I didn’t use it on a book cover or a t-shirt. I didn’t put it on an album cover or run it behind a paywall. I didn’t sell the image and I didn’t make any money from it. It wasn’t like these were the days of the wild, wild internet west and I simply didn’t realize there were laws governing the use of other people’s photos. In fact, I thought I had a pretty good handle on what did and did not constitute fair use (I did not) and that since my site wasn’t monetized at the time of the post and the photo was used in part as a parody, I was in the clear."
"Also, the way I used the image did not fall under fair use. Though Steve Forbes was being made the target of a joke (or parody, to get all fair use-y), it wasn’t Steve Forbes suing me. It was the photographer, suing because I had used his work without permission. If I had altered the image or copied the image in a parody of that particular photographer’s style, that would have been a parody, but being a photo of the subject of a parody wasn’t enough to make it fall under fair use."
I think this means there's a difference, fair-use-wise, between using Buffy screenshots to make Buffy artwork (a transformative work of the same work the screenshot is from) and using other images to make Buffy artwork (not a transformative work of the original photographer's/artist's work).
Obviously, all this is more important for people who ever plan to have one of those Patreon/Ko-Fi/whatever thingies.
The comments section is interesting, too, and includes a link to this older post by someone else (Roni Loren):
Bloggers Beware: You CAN Get Sued For Using Pics on Your Blog - My Story
Both include some great tips on sourcing images.