It might have something to do with the image's resolution, h/w, or even the file size.
Changing the image's filename alone has always worked for me though.
...speaking of Topic Images...
I just realized that one of my topics (
RideBack) just lost its "topic image" all of a sudden! :? - I'm very certain that I did include an image upon creating it, as every Anime topic that I've made has. (so my question is, where'd it go?) :?: