For forum related questions, post here. For GFX help, post here. Also, could you please clarify? Click on 'User CP,' then click on 'Edit Signature," after that click on this image: and paste in your signature's URL.
A common mistake people have is trying to upload the signature to the website directly. The image file size may be too big, you should try uploading it to somewhere like tinypic.com or imageshack.us and putting the