I have a suggestion and/or request:
The Sony 600 with PRS+ now supports covers, but it uses in an entirely different directory. I wouldn't be surprised this will be the only new wallpapering device to pop up with this problem.
If there was a user field to enter the path string, it could become a user option -- perhaps with "default" buttons that fill it in for known device paths. The auto-sensing is nice, though.
Keep up the good coding!