As far as I can tell, it's not just an issue with profile pictures and banners, it's an issue with all image uploads. I've tried a few different times to upload an image as part of a post, and I get:
SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
Seems to be the same on both Firefox and the Jerboa client (although Jerboa doesn't show the text of the error, it just silently fails).
So if this isn't a high-priority issue, maybe it should be. Unless other folks are able to upload images as a part of posts and comments? If I knew any JavaScript or webdev I'd volunteer to help, but alas I only really know Python.