If you have access to a Macintosh, it's even easier. Download a program called GraphicConverter (shareware, so it's much more affordable or you can just use it once), and open your image file in it. Choose 'Picture->Size->Scale' from the menu. In the window that pops up, make sure Width and Height are measured in Pixels. Then, double-check that the 'Keep Proportions' box is checked. Type '64' into the Width, and it should automatically set the Height to the right proportion to match. If the resulting number is greater than 64, then just type '64' into Height instead, and it will make the Width the right size. No fuss, no muss.
When you 'Save as...', you'll have the option to choose file type. Choose JPEG, and click OK. You'll get another window with a slider, which lets you set the compression you want. Slide it down until you get a file size less than 5k, and save. Done!