This is a great idea. I love it.
But it would be even better (IMO) if the thumbs panel
also had a mode a la the Picasa browser, wherein
all thumbs for
all folders are displayed in a scrolling list grouped by folder. Selecting a folder in the tree centers that folder's images in the thumb panel. This is the most powerful image browsing metaphor I have encountered.
A combination of these two modes would be killer.
Note: Picasa's view is very similar to the image
Kirukato posted above (just missing the checkboxes in the tree). Here's a look:
