In Photoshop, Image > Image Size.... I don’t know GIMP, but I suspect there’s a similar command. Picasa will also do it, and there are probably hundreds of smaller apps that can do the same thing. If you’re using a Mac, head to MacUpdate and do a search for image editing software. I’d be very surprised if there weren’t at least a half dozen similar websites for Windows, Linux and Android software.
Most websites prefer images that are under 1MB in file size for background images, and many prefer images in the 50-100KB range for user icons. dA, for instance, uses a 50 pixel by 50 pixel size for user icons, which you can easily make under 10KB.