on the front (home) page of pregame lobby there's a link to
MyGamerCard.net.
- Click on it.
- enter your gamertag and select a style that you want to use from the gallery.
- now click "Generate Gamercard Code"
- Click on Forums (Image)
- copy everything in the box labeled URL: (it should say sopmething like "http://card.mygamercard.net/gelsig/gamertag.png" where "gamertag" is your gamertag.)
- Move back over to PGL. In your User CP, under Settings and Options, select Edit Signature.
- A box just like a posting box will appear.
- Select the Insert Image icon (looks like mountains with a sun)
- paste the URL you copied into the box that pops up.
- You're done!
Good Luck!