Maybe your using the wrong format, try using either .png or .jpeg
May be be that the image you made (If you made one that is) was saved as a .psd , .xcf (gimp) or a .gif (animation, which doesn't work for user without tp or vip)
Is it a very big size image? That might be the problem.
Forum avatar system automatically scales big images to 100x100 so that's not a problem.
For now, try this:
Open the file in MS Paint. Go to File --> Save as.. then choose png extension from the drop-down menu. Upload the .png files of your image. This will most likely solve the problem.