If there is an API tag that shows the resold account name (e.g. username for resold accounts, root for all regular ones) then I suppose you could have a logo file with the same name as that API tag..
For example: If I have a reseller called &john& and the other accounts are under root, I could call my regular image root.gif, and the image for the resold control panel john.gif. I'm not sure if there is an API tag for this, though...
Good luck,
Brian