Here’s a useful graphics tool that allows direct measurement of image dimensions on a computer screen. It works just like a pair of calipers that you can move across the screen to any position and then open and close to measure the length of any portion of an image. I find it much easier to use than the on screen ruler that most graphics applications provide. It works with any graphics program. There is a free version that omits some more advanced features but is very useful anyway. The tool can be downloaded at: http://www.iconico.com/caliper/