Thats a good question, Galen is really meant to use for importing pictures into Palace no the other way around.
But it is possible to do this anyway, what you need to do is the following:
wear the prop your trying to edit and paint the background green or whatever color is the biggest difference with your prop.
Now you can press
ALT + PRINTSCREEN on your keyboard and the picture is loaded in your clipboard. Make sure you use the ALT together with the printscreen, this way you will only capture the Palace client window and not your entire desktop.
In Galen you can import the picture by pressing
Clip->Main
Naturally you will have to locate the prop in the whole picture wich will be a lot bigger then an avatar.
good luck!