Person 1: "Singnatur"
Person 2: "Whuat?"
Person 1: "Signatur sry for bad english"
Person 3: "i got a feeling this is in the wring section? doesnt it tell u everything about the signature on the edit profile thing?"
Person 2: "Wtf?"
Google images > banner *what you want in the banner* > Select the image you want > right click with your mouse on the picture > Properties > Adress(URL) > Copy the image's URL > Edit your signature > Paste > put [-img] in front of it and [-img] at the end(Without the "-" part).
Was that hard?
Edit: If you want an banner from your computer, just go to Tinypic or Imageshack and upload it. Then edit your signature with the IMG tags