My comic, Jack and Deneez, is rated M and categorized under Horror, but the thumbnail is showing E and Fantasy. It's not a big problem at the moment, since I haven't gotten to anything that needs the M rating yet, but it'll come up in a week or two. I've tried changing the rating and category and then changing them back, but neither of them show any changes.

Yeah, for some reason the first thumbnail always defaults to E. Try uploading a different thumbnail (or even the same one with a different filename) and see if that does it. You might have to tell your page to refresh to show the change, it might try and show you the old image from the cache still.

That fixed the rating, thanks. Category image still shows fantasy, but that's not a big deal.