TRANCE  nice for an initial version.

my suggestions:

delete"d" saved thumbnails spelled incorrectly.

keyboard left and right buttons should go back and forward in the folder, not move the picture around the screen.

need a trash/delete button.

doesn't remove cached thumbnails of images that have been removed from folders.

thumbnail location is wrong, should be in application support instead of caches because they will get deleted every time the caches are cleaned thoroughly.  
(Version 0.1)

DRDUL  Crossbow is a promising app. Since switching from Windows, I've been looking for a well-designed, easy-to-use image browser. I've tried 'em all, and I like Crossbow the best so far, which is particularly impressive given that this is only version 0.1. I'm looking forward to future releases.  
(Version 0.1)