It depends on what program you have to view pictures in. When you open a picture, does it come up in just a viewer (with no menu at the top) or in a program with a menu? If you see a menu, look under the options in the menu and find "Resize". Then resize the picture. You should be able to view the height and width in either inches OR pixels. Change it to pixels if not already there, and then change the largest (height or width) to 500. The other will change automatically and keep the correct proportions.
You SHOULD have a picture program, but if not, and you only have a viewer, right click on the picture and click properties. Then click the "Advanced..." button (if you have Windows XP anyway, not sure with the older versions), then check the "Compress contents to save disk space" box and click OK.