Does the image in question conform to the HJU SIG GUIDELINES? If it's too big, that could be the reason behind your problem.
Go here to read the rules and see if the image conforms to what is permitted:
http://forums.henshinjustice.net/showthread.php?t=16707 If the image DOES conform to what the Guidelines allow, then do this:
1. Go into your
UserCP, once there,
2. Click on the
EDIT SIGNATURE link found on the left side of your screen
3. In a new tab, upload the desired image onto your Photobucket/ImageShack/TinyPic/Flickr page.
4. Copy the appropriate link that will DIRECTLY link you to the desired image, and paste it in the "Edit Signature" window, making sure to place the link within the "IMG" tags.
5. Save Signature
If THAT still doesn't work, just post the image that you wish to have in your Signature here, and one of the staff will set it up for you.