Hmmm.. Never heard of that issue and not sure why you'd be having issues. That's definitely not related to JFBConnect, but I can try to help however I can. First, I went to the
bug-reports area
of Facebook to see if there have been any recent bugs about images and apps. I couldn't find anything.
Then, I went to one of our test apps and uploaded a 200x200 PNG (15kB) big as the main image for the app (the big image at the top next to the icon). Facebook accepted and updated the image immediately. I used Chrome on a Windows PC to do so.
So, it seems to be working for us, and I'm not sure what could be going wrong with the upload process. If possible, I'd recommend trying from a whole different computer as anti-virus or other addons on your computer could be preventing the upload of images for security reasons. It would be odd, but we've seen stranger things happen...
Hope that helps, but if you continue to have issues, let us know. We may only be able to help you submit a bug report to Facebook on the matter though.
Best of luck,
Alex