Could this be the problem?
Signature Permissions
Allow Basic BB Code Yes
Allow Color BB Code Yes
Allow Size BB Code Yes
Allow Font BB Code Yes
Allow Alignment BB Code Yes
Allow List BB Code Yes
Allow Link BB Code Yes
Allow HTML No Allow Image BB Code No
Allow Code BB Code Yes
Allow PHP BB Code No
Allow HTML BB Code Yes
Allow Quote BB Code Yes
Allow Smilies Yes
Can Upload Images for Signature No
Can Upload Animated GIF for Signature No