To expand on what Suevans posted, the two image files should go in the top of a themes folder. The address will then look like this: /var/stash/Themes.../
[AnyThemeName].theme/
[those two png files]
As long as whichever theme you have the folder in is on top (in Winterboard), those two files will override the stock SMS backgrounds.
Oh, and the sizes should be:
SMSBackground.png = 480 x 480 pixels
[email protected] = 960 x 960
So, one's for the retina displays, the other is for the non-retina displays.
I have a lovely dark wood background in my SMS app (biteSMS). Works well there and in the stock Messages app.
Marilyn