I can only de-associate Iceweasel and gThumb from a JPEG, GIF, BMP or PNG by manually editing the MIME types database.
The Remove option to those entries is greyed out in the Open With section.
Now the weird part: If the MIME types database is updated, say I install/remove an application like File Roller, Iceweasel and gThumb are re-associated to the image types.
Another one: If I rename the Iceweasel and gThumb menu entries, I can remove their entries using Open With option. Then their menu and MIME entries are immediately recreated/reverted to their original state.
What could be causing this behaviour?
I'm on a custom Debian Testing AMD64 install and I used the latest (RC2) netinst installer.
I started with gnome-core and kept building my system from there.