Resize Image

I have an .asp page when you click on one of the products it takes you to another .asp page and displays the product details for this product. This product details page is the same page that displays any product you would like to see more details of. The problem I have is that each product image is a different size and ratio. I need some type of script which will not force the image to be a certain size but restrict it from being any taller than 396 pixels and no wider than 518 pixels. If it has to restrict the width then it should conpensate for this by makeing the height shorter to keep it in proportion. And vice versa. But if the picture is smaller than both of these measurements then it should appear on the screen it's full size. If you know the answer to this question then please bid for it. But if you cannot solve this problem straight away then please do not waste my time. I need a quick solution not somebody who is going to take 3 days to write a script.