NexusImage is a lightweight, portable and easy-to-use image viewer.
Launch the program and you can browse straight to an image in the folder you'd like to view. At this point we weren't too impressed; file format support here is limited (jpg, png, bmp, gif, ico, tiff, wmf, emf, jp2, j2k), and there's no Explorer-like thumbnail browser. Once you've chosen an image, any other pictures in the same folder are displayed in a scrollable strip on the right-hand side of the screen, but this is only really useful when browsing folders with small numbers of images.
Get past these issues, though, and NexusImage does have a few points in its favour. The program can rotate and zoom in on a particular image, for instance, but a "smooth stretch" setting helps to minimise any pixellation or image artefacts as it works, so your pictures look better even at the highest magnifications.
Open the program's Exif window and you'll be able to view the metadata of each image as it's displayed.
There's an option to set the current image as your desktop background. You can add a caption to an image to help you remember something about it (this doesn't affect the picture itself, it's just for use in NexusImage). And there are shortcut keys for everything, so once you've mastered these then the program is very quick and easy to use.