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