Support for non clean URLs

Thanks for this great module.
My setup uses private download (for security reasons) and no clean URLs (because my provider doesn't allow it yet).
The private download part is working, however, in conjunction with non clean URLs, no images will be found.
Attached is a patch that fixes this problem.
Please take a look. Also, being still rather new to drupal developement, leave comments if my solution is OK or what could be done better.

BTW: I patched against the latest HEAD revision of Image_Resize_Filter