one of the few programs I really missed in Ubuntu. But I just discovered the portable version works great under Wine!!
And as I 'm not a purist (sorry)...
FastStone Image Viewer is another great app from FastStone which is one of the best image viewers in my opinion. This little free application which also comes in portable version is a user-friendly image browser, converter and editor, all in just 3,9 mb.
FastStone Image viewer has all the features you will need, like image modification tools: Resize/resample, rotate/flip, crop, sharpen/blur, brightness/contrast, red-eye removal, emailing, resizing, cropping, drop shadow, framing, bump map, lens, morph, waves, color adjustments and digital camera support but what i like the most is the built-in batch processing (you can rename or convert multiple images in the same time).
Unfortunately, FastStone Image Viewer comes only in a Windows version, but if you really want to use it on Linux, the portable version (downloadlink) works great through Wine.