I dunno whether this is really a bug or an intentional change that was made recently (I only noticed the issue myself a couple of days back), but I can't upload series images that were edited using Adobe Photoshop.
If I don't edit the picture and just upload it as-is, there's no problem. But unedited covers rarely look nice, especially with the resizing that happens automatically.

I've tried using all 3 of the allowed formats (JPG, PNG and GIF), but the image doesn't change. After I click 'Update Series', I either get a blank page or am directed back to the series page with the image unchanged. I tried several different series but the result is still the same, so it can't be an isolated problem.

Sounds pretty insignificant, I know. But it's kind of bothering me, so...

I had this problem myself a few hours back. I didn't try as much as you seem to have, but the Photoshopped picture I tried to upload wasn't accepted (the format was PNG, which is what I normally use when adding series images to MU), and a blank page turned up.

Edit -- And now I decided to give it another shot; it works fine again, at least for PNG (dunno about the other formats).
@cyclamen -- still no luck?

cyclamen, is this still an issue?

Yay, it works now!

Sorry for making an (unnecessary?) fuss 😳