I've posted a test release of a portable version of Cornice, a simple image viewer. It can view thumbnails, rotate images, zoom in and out, do slideshows and more.
I'm posting this before it's release-quality as a couple people asked for it. There's a bug in Cornice itself where it interprets single clicks within the thumbnail view as double-clicks unless you click very quickly. I finally figured out the issue today as it apparently immediately goes into drag mode on mouse down... unlike in detail view where it doesn't. And when you drag and drop an image in the right pane, it opens it.
Also, the launcher will automatically use your X:\Documents\Picture directory but it doesn't yet remember other directories. I replaced the existing old-school icons with some custom ones I created from the Nuvola icon set as well. Oh, and it works on Windows 95 through Vista as well as Wine within *nix.
Please give it a try and post any feedback.
Download Cornice Portable 0.6.1 Pre-Release 2 [4.1MB download / 5.8MB installed]
And, if anyone knows Python well enough to fix the click-drag bug and recompile it all into a stand-alone EXE, I'm all ears.
UPDATE PRE-RELEASE 2 (August 22): Got a quick patch from the author that fixes the drag bug in thumbnail view. Also updated the launcher to remember your last used path and adjust it as you move between PCs. So, it's now a full-fledged pre-release.