So I go to upload an MP3 render for my game jam entry, right? But when I try to do the thing, it doesn't work, it says I can only upload an image. But the jam rules say I should upload an MP3 render? ("All tunes will be rendered to MP3 for voting accessibility.")

Those rules are not the jam rules. They are rules featured by default in any battle. So, you really are supposed to only upload an image preview of your entry as far as the tic80 format is considered.