It would be great if:
1) thumbnails showed the actual entire image, not just a square crop of the image - it makes it harder to tell from the thumbnail what the image is. Just do it like Mac Finder.
2) user could change the size of the thumbnails using a slider (again, like Mac OS)
3) user could hit the space bar (or similar) to quickly preview an image full-size, then hit escape to return to the thumbnail grid (again, like Mac OS Finder window in icon view)
4) the window didn't resize itself every time an image was opened, based on the size of that particular image - this is weird and annoying, especially since when returning to the thumbnail grid, the layout changes so the user loses their place / context because everything has shifted around
If these things can't be improved, then it would be nice to be able to just right-click and choose "View in Finder" and browse all the images that way.