Everytime I see it I'm taken aback. There seems to be no pattern between them but some pictures still bypass FA's maximum image DPI.
FA has a few known exploits ... that is one of them. Apparently, and I haven't tested it, it's easy to break that limitation as it only checks on the initial upload.
1. Upload new image that meets the max image size.
2. Edit submission and re-upload the image, but the over-sized version
3. ...
4. Profit?
AFAIK that's it. I'm not sure if they have started enforcing the restrictions or what, but the preview versions are always the same scaled size.
Sounds like something that'll work.