The way I do it is create the large picture, save it as a pdf file and then
on the website create a link on the small picture to the pdf file.

Take a look at my site: www.advancedaudioinc.com for my solution.  I created
a group of thumbnails on my "Project Photos" page.  I then created a
corresponding page for each picture and placed a large version of the picture
there.  You then hyperlink the thumbnail to the page with the larger picture.
I then hyperlinked the larger picture back to the thumbnail page.

There are ways to launch a smaller window with a larger version of the
picture.  I dont like this as you end up having to close that little window.  
I think my method works better.

Good Luck!