Broken category thumbnail images?

Just what it sounds like. My category images will randomly show up as broken. No changes made to the template, no moving of images in the file structure, no provocation whatsoever. Some will show up and some won't. Some will show alt text with a broken image and some will just show a broken image. What's extra odd is that it seems to fix itself after an indeterminate amount of time.

The issue is also fixed (briefly) when I reassign the same category image thumbnail in the Content Manager, but then seems to reappear when navigating to another page and then back again. Some images show up consistently, while others are consistently broken. However, I can not find any rhyme or reason as to why some show up and some don't (color space, files names, etc). I've tried downloading one and uploading with a new file name, too. That seems to have helped on the image that I applied it to, but I really don't want to take the time to