Interesting trick you can do with any graphic. A graphic can be displayed at any size, regardless of it's original size. This page demonstrates this concept. Below are 2 images of the same motorcycle. One is originally 343x257(big) and the other is 153x116(little) in pixel size. You can resize either one at your desire. Try it. At some sizes both pictures look the same. View source to see how this was accomplished.
To those that haven't figured this out yet...You can use the same image in different places. Instead of multiple picture files you can incorporate something like this. You could provide the available display sizes from a list for that "Zoom" concept. See lot's of sites that would benefit from this concept. As a rule you will find larger pictures will scale better than smaller ones.
Remember you don't need this fancy stuff to use the concept. Just change the width and height properties of your picture to size it to what you want.
This can been taken to extremes and cause problems. An example of this would be to create a large image and then display it in smaller 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).