Here is a theme for your SMS background. If you'd like to use your own wallpaper just use ifile to replace the "SMSBackground.png" image within the SMSStripes theme folder. Or if you dont wanna download this you can just go to your themes folder and create a folder named "SMSBackground.theme" and place your wallpaper that inside. The name of the folder can describe the wallpaper as long as its followed with ".theme" but the wallpaper MUST be named SMSBackground. Also in iOS5 there are grey lines around the picture when you receive an MMS. I havent found anything on fixing that though. Not a big deal to me. Hope this makes sense, it will to people familiar with using themes and ifile.
Download SMS Background Theme
**SMS chat bubbles not included, still editing from a theme from deviantart**