There's
Microsoft's Image Resizer Powertoy for XP, which lets you right-click on an image (or group of images) and resize to a custom size.
If you want something a bit more batch-oriented, there's
ImageMagick for Windows; it's a command line tool for image manipulation. Does a lot more than resize, if you want, but can be pretty hard to use if you're not familiar with the Windows command line.
Neither program is specific to the BlackBerry or Desktop Manager; the images would have to be resized prior to uploading.