OK - so are you asking about your signature box?
That is done in CP like Carol said under edit your signature. There is a good description on that in the FAQ's.
You need to paste the link or html and for mine at least, it would say it didn't work but it does. You are limited to the spaces. But I haven't figured out how some have posted their blinking, etc. additions such as the Cheetah ladies do.
ETA : Yes, I just added the Sharks "stuff" to mine. Just go to edit your signature and upload either a web link or a jpeg from your own computer. You can add any typing there also and change colors, fonts, size, etc just like when posting a message.
Have fun !