Not everyone knows that cds or usb drives appear in /media ("normally" they
appear on your desktop!). In gthumb it is almost impossible to find these drives. 

My suggestion is to provide gtk(nautilus)-style folder pane (with mounted
filesystems and bookmarks), or add a possibility to display this folder view to
find more difficult locations. Favorites do not provide this functionality (they
are "favorites" while cds or usb drives are generic). 

Of course, you can open files directly from cd or usb drive, but gthumb suggests
it is also a browser, so I've noticed that people tend to open it first and look
for images later. 

Most of the cds I get from photo labs (well, there are still people scanning
films) do not work properly with linux (it is obviously not linux' fault),
meaning gthumd does not propose to open images. And I have never seen this "open
images" dialog for UBS drive anyway. 

Other information: