Comment 13 for bug 351122

> Why doesn't %U expand to something more specific than ~/.gvfs?

Sorry, I made a slight mistake, %U actually seems to expand to: ~/.gvfs/gphoto2

I know this because if you change that Exec line to:
Exec=sh -c 'xmessage %U'

you will get a pop-up upon camera insertion telling you to what %U expands. (~/.gvfs/gphoto2).

To show that such a directory path is not sufficient, if you plug in the camera and let it mount automatically and then run this command manually:
sh -c 'gvfs-mount -u ~/.gvfs/gnome2'

you will get this error message: "Error finding enclosing mount: Containing mount does not exist"

For reference, at this running on my computer, "ls ~/.gvfs/gnome2" shows the following:
gphoto2 mount on usb%3A001,007

So that's why I made the above suggestion to use the following command in the .desktop file to unmount the camera in order to make sure the full path of the camera is specified:
Exec=sh -c 'gvfs-mount -u ~/.gvfs/gphoto2*; exec gthumb --import-photos'

Also, as another/additional problem, the reason I removed the double quotes is because if you change that line to this:
Exec=sh -c 'xmessage "%U"'

you will not get any popup what-so-ever because the double quotes seem to prevent that line from executing.

So my conclusions were:
1. The %U is not expanding to provide enough information to unmount the camera.
2. The double quotes seem to cause a problem.

Disclaimer: I am not an expert. There may well be a better way to fix this issue. But hope this helps explain my logic behind the above suggestion.