Here are a couple of ways to do what you want (I guess).
An old trick using tables is to position a 1 pixel transparent gif inside of a cell, scale it to the size of your image and put your real image as a background. If someone click and drag they'll get the transparent gif.
If you use CSS, you can create a div, give it an ID and write rules for the size and the background image.
People can always take a screen picture and paste it in paint, but, at least you'll make them lost a couple of minutes.