(Noob question) Editing the conversation window background image in PST:EE

After two days noobingly trying to understand how to edit BIF and MOS files (Using stuff like WinBiff, MOSutility, MOSWorkshop and the likes) i have started to question my sanity. Can someone help me out here...

All i want to do is to edit one image (The background image of the conversation window) and then toss it back in the game again and then be happy forever.... But i can not find this particular image anywhere!

I have changed the ui a bit and now want to do this last change. Since a screenshot says more than a thousand words take a quick look here and you will see exactly my point.



Thank you.
Manglu