It's that "attach file" button. That's never been there before on the 3D contest and I think it is what's causing the problems. I had no idea what was supposed to be attached there, so I added my image file on both the "Image" and the "File" lines and it finally took it. Make sure your image and thumbnail are both at exactly 72dpi too - my thumbnail somehow got saved at 72.009 dpi and it kept getting rejected.

Glad to hear you figured it out. As Gilda Radner (RIP) used to say, "It's always something."