I'm not a pro on GFX, and the GFX section on sythe's forums would be a better place to ask your question. But could it have anything to do with having a file extension that doesn't match the format of your file?
Once you make the picture etc, save it as any extension, post it up on www.imageshack.us or tinypic. Then get the direct link to it and to put it in your profile put [ CENTER ]
If you can't upload it to the Sythe server then you can try upload it externally then link it or the reason you can't upload it to the Sythe server is because the file extension may be incorrect.