when you are making the signature, say you make the size 400x150.
now the next thing you would do is make the bg only about 300x100 inside of your 400x150 work space, and make the area behind the 300x100 work space transparent.
next pull in your render and make it bigger than the 300x100 area (going into the 400x150 area) so that it apears to be popping out of your 300x100 area signature.
then save file as .psd, then save a copy as .png. that way the bg will stay transparent when you use it on the interwebs
