Interesting trick you can do with any graphic. A graphic can be displayed at any size, regardless of it's original size. Once you know that, creating a Zoom function for a picture is simple. Although the original display size for this picture is 50x40 the actual picture size is 343x257. In this way we can scale the picture from 50x40(1x) all the way to 600x480(12x) with good quality. Higher zoom levels(16/20/50x) degrade the image noticeably. Best of course is "Real Size"(6.8x). This is quirky in Netscape, the picture resizes but does not display. Requires version 4+ browser.
View source to see how this is done. Very simple function. Again the concept is important. You can adjust this for your specific needs.
Warning!
This can been taken to extremes and cause problems. An example would be to create a very large image and then display it in thumbnail sizes. Suggest you find a middle ground. An image that is 300x200 can scale anywhere from 50x40 up to 600x400 and still look pretty good. You can usually "Zoom" any picture to 3x original size with little quality loss. Just remember, using our example 600x400 would be 4x original size(think about it).