Move unnecessary stuff out of update_image():

Situation: On every call it was checked if it's an animation, picture or if this stuff was already checked.
Normally that should be checked just once. If move_index() is used. Only time self.pixbufOriginal gets a completly new file.
So the check if the loaded animation is really an animation or just a static image is moved to move_index().
In update_image() just remains the check for a pixbuf or pixbuf animation as they're differenly set.
No zoom with PixbufAnimation

pygobject doesn't support scaling with a PixbufAnimation. So if checks included to ignore zoom_delta and zoom_to if it's an animation.
0.5.2
Optim handling of corrupted images

If new_from_file() returns None the picture is probably corrupted (also could be something else. See doc).
If None is the case, the path will be ignored and the index will be moved up or down (whatever value delta might have).