rajkard: Batch renaming of images might be useful too, eg. give a prefix and it will append numeric suffix to make each file unique, so you could take a selection of images and rename them all to be easier to sort/catalogue locally. Let us know how that project comes along! :)
There is a great and now free app for that: http://www.rlvision.com/flashren/about.asp (created by game developers) Beats any other tool when it comes to mass renaming stuff with templates in just a matter of seconds to setup. But yeah some nice rapid fast lightweight image viewer that is borderless and can stay on top…