here is how "enlargeable" images on webpage are "usually" done.
typically you have 2 pictures.
you have your original picture, and then you make a tiny version of it (a thumbnail).
then on your web page (or in your ebay listing) you would do something like:
<a href="mypicture.jpg"> <img src="mypicture_thumbnail.jpg"/> </a>
it can be done with a single picture as well, but it's typically done using 2 pictures, one large one and one thumbnail. so that html code above shows the thumbnail picture "mypicture_thumbnail", and if they click on it they will be shown the main big picture "mypicture_thumbnail.jpg".
i don't know to what extent the ebay listing system helps you with such things.
i know there are shareware programs to help you post listings.
maybe there are freeware tools also.