Alright, things just got a little twisted.
Turns out this whole "everything can only be saved as .bmps" only applies to forum avatars. I just went into the Forum Waiting Room to grab a picture on IE8, forgetting that I can only save as .bmps until the last second. I decided to try saving the picture anyway, and it did, in fact, save as a .png. I tried to save my avatar after that, and it would only save that as a .bmp.
Maybe this is a problem with the forum and not IE? Does this have to do with the fact that Badspot fixed avatar uploading?