You could also download this freeware: www.irfanview.com. You go to "File", and "Open" your image. Then go to "Image" and "Resize/Resample", which gives you a number of options, the easiest probably being clicking "640 x 480 pixels" on the right-hand side. Once you've made the changes, go back to "File" and use "Save" or "Save as...".