Update:
This can all be accomplished directly in gmail now under Settings->General->Signature
Those of us that use Gmail know that adding an HTML signature can be a pain in the behind. Typically, Gmail only allows standard plain text signatures. I have discovered a great Firefox plugin named Blank Canvas Gmail Signatures. This plugin allows you to easily add signatures to each sending address within your account.
Simply compose a message and click “Create Signature”. Once you have done that for the sending account that you want, your screen will look something like the image on the right. From now on out, any email that you send from Gmail on that computer will have whatever HTML signature you want.