Seeing as I just set this up, I should note down what I did.
Max upload size is too small
I need to upload lots of big images so fixing up the upload size limit was a first step, this is the advice I used: https://www.kasareviews.com/fix-upload-max-filesize-wordpress-error/
upload_max_filesize = 20M post_max_size = 32M memory_limit = 128M
The reasoning being that: the biggest file will be 20MiB, it'll be included as part of a POST body that could be larger because it has to encapsulate it on the wire (and in theory there could be multiple images in a single request), and PHP needs enough memory to receive and process the file.
Yeah look, I just don't want comments. I don't want to deal with them, see them, antispam them, etc. It's not worth my time. This is a place for me to dump content that doesn't suit twitter and is irrelevant to shortgirls.net, and needs richer presentation than my wiki affords, so I'm not interested in taking comments.