Image resizing should be a simple problem to solve, except that it seems like everybody and their dog--sorry, cat-lovers--has a lightweight, freeware image editor out there. For example, there's Image Resizer. The name is simple, the installer weighs in at a rail-thin 357 KB, and it's free. Can't argue with that, right?
Except that it barely works, at least when I tested it. The interface would be fine, back when Windows 3.1 was new. It's nothing less than a giant pain now that we have such cutting-edge improvements as long filenames. My attempts to convert JPEGs en masse and one at a time were met with errors. If it can't handle the most popular image format around, it's not really going to be worth downloading.
Instead, I'd recommend going with a tried-and-true favorite such as the free FastStone Photo Resizer. Able to handle far more than JPEGs, I've used FastStone regularly for batch resizings and image conversion. In fact, one of the best things about it is that it can handle both on the fly and simultaneously. The interface is a little bit cluttered and a departure from boilerplate offerings that look like descendants of Windows Explorer, but it doesn't take long to figure out.
Output formats include TIFF, GIF, JPEG, and BMP. Besides batch-conversion, the application also can batch-rename pictures. Users can even make basic tweaks, including cropping, rotating, resizing, and simple color adjustments. You'll even find a tool for stamping shots with a watermark.