Use this thread: <a href="http://www.newgrounds.com/bbs/topic/1082368">http://www.newgrounds.com/bbs/topic/1 082368</a> - The Official Art Portal Q&a Thread
The only thing I can think of is either if the filetype is wrong (although it does not say what files do work) or if your uploading speed is too slow/you click "submit" a bit too fast (the pic has not been uploaded to NG by the time you press "submit", if it´s like other pic uploading stuff on NG then a "scanner" should run behind the text until it has been loaded).
Amereep
Within the Image category click on the browser and navigate to your picture. (All of my pictures are saved as Graphics Interchange Format (GIF)). I hope that I was of some help.
KaboomKid
Thank you, this helped a lot and I have now uploaded some of my work.