I'm not really sure I understand the need for a "skinnable" image viewer. But anyway...
Kuickshow is pretty good if you're browsing through a directory with a lot of images. It will display thumbnails and you can find a particular one fast. It also has slide-show capabilities.
xv is fast, and easy to use. It's a little primitive looking, but gets the job done.
Those are just two that I use. There are many others. Try browsing
to see what else is out there.