It was just the letter case - I had ".jpg" but the files had all been exported as ".JPG".
(Somebody on another a forum helped me out - they pointed out that if it was working on my computer but not on any host (I tried Itch and another one), then it was very likely a case thing, since Windows typically isn't case sensitive. That's when I noticed I didn't have my file extensions switched on!)
Thanks for your help!