The easiest way is to put a bit of code that looks something like this into tpl_product_info_display.php:
Code:
<?php if ($_GET['products_id']=='7'){echo '<div id="newimage"><img src="Path to Image"></div>';}?>
You need to change where it says 'Path to Image' to be the path to the image. And change the number '7' to the product id of the product you want the image to display on.
You can put it wherever you want the image to appear. If you want it under the add to cart button you will put this line just below:
Code:
<!--eof Add to Cart Box-->
You may have to put a rule in the stylesheet to control its display too but I don't know because you haven't given us a URL which is always kind of useful to stop us having to guess!
It might be something like:
Code:
#newimage{
float:right;
clear:both;
}