well since its going to be something that generates a simple skin I would think about having the following things to be able to edit:
Sign In Window Background
Add picture on top of background in sign in window (requires adding new element to UIFile):
Contact List Background
Dashboard Background (would apply to other windows as well)
Add picture behind contact list (kind of like lv back)(requires adding new element to UIFile)
Chat window dashboard background
Chat window default background
Alert background (sign in and large alerts)
Contact card heading background (requires adding new element to UIFile)
That's off the top of my head, sorta looking as to how the new beta affects skinning atm so I am a bit distracted
The UI I would imaging as just having a screenshot of the window you are applying things to and then options down the side.
/ 
