Thanks for the replies first off.
I'm not sure though if I was being clear enough in my original post. What I wanted was custom backgrounds on posts that everyone can see, but is only displayed in certain usergroups. An example of this can be found here:
http://sleepywood.net/forum/showthread.php?t=1617795
You can see that a user has a pink background on their posts with a little signature image almost in the right hand corner (the post text will overlay this if the post was larger).