Xnview

There are loads of image previewer/thumb nailing applications from some web based - even Konqueror and Nautilus have some of these features.

So why do I pick Xnview?

Its ugly. It uses old, Motif type libraries, but it has two very redeeming qualities:

1. Its very fast, as fast or faster than some similar proprietary apps.
2. It handles a wide array of image formats with very good conversion capabilities. There are some real obscure file formats Xnview can open and convert. It's probable that only Imagemagick can handle more. I've found it very stable and worth at least adding to your toolbox.