Go to edit signature and use the tag
CODE
[url=http://..][/url].
around the image, for example if you wanted your i joined the archangels to link to www.archangels.com like this:

it would be:
CODE
[URL=www.archangels.com][IMG]http://i5.photobucket.com/albums/y185/MB_Bounder/good3cr.jpg[/IMG][/url]