Go to
PitaPata and follow the instructions they give you to create the banner. Then go to CP, as Ron says, to Settings & Options>Edit Signature.
You may have to try different code options in PP to get one that looks right. I think that Forum signatures>BB Code worked best for me. So don't close out the PP window until you try different ones in BH and are sure you've got one that looks right or you'll have to go back and do it all again.
Cool site!