Maybe this is already known, but I haven't been able to find this solution anywhere else.
Ok, so I've been theming my phone with nothing but Megan Fox(oh yes) and one of the only things I was lacking was the sms background. Since I have BiteSMS, it's not as simple as dropping an SMSBackground.png file into the root of your phone. I've looked for quite some time, and had found no solutions. I had recently downloaded a Marissa Miller SMS backround from Cydia and it worked like a charm, within BiteSMS. Hmmmm. I navigated to Library---Themes---MarissaMillerMessages and I find what I'm looking for: SMSBackground.png. I changed the name of the folder to Megan Fox SMS and dropped my own SMSBackground.png file(resized to 320x375) into the folder(replacing the existing one) and then jumped into my Winterboard. At the top of the list was sitting Megan Fox SMS. Avtivated it, resprung and voila!! My own personal BiteSMS background. I apologize if this is located somewhere else, but like I said, I couldn't find it anywhere. So I thought I'd share.