please read & comprehend my post, it does in fact work for ANY MEDIA FILE ANYWHERE ON THE XBOX, provided you actually follow the instructions...
-EDIT-
i feel sorry for you, so i decided to post examples from my own soundtracks (which all have tbn files because i kick your ass).
ALL of your soundtracks will be found at E:\TDATA\fffe0000\music\, inside numbered sub-directories (one for each soundtrack). i have one in a directory named "0000" which i will use as an example.
inside of E:\TDATA\fffe0000\music\0000\ i have a bunch of WMA files with names like 00000001.wma, 0000000f.wma, etc.
to do a specific example lets work on 00000001.wma, which in my soundtrack is A Perfect Circle-Judith (Renholder Mix). i got this track from the Underworld soundtrack, so i wanna use that cover art as the icon.
here is a suitable (if a bit large) graphic file:
http://ec1.images-amazon.com/images/P/B0000AYL3Q.01._SCLZZZZZZZ_.jpg
the actual file i am working on (once again), is found at:
E:\TDATA\fffe0000\music\0000\00000001.wma
as a result, i need to upload & rename the above graphic like so:
E:\TDATA\fffe0000\music\0000\00000001.
tbnit's not that hard, people...