Here is one way that I've have accomplished this:
Let's call your image file, pix.gif
Using Irvanview you can change the size of the picture to the thumbnail size
of choice and a second file to the size to displayed when the thumbnail is
clicked on.
On the server create the folder, downloads
Upload the enlarge pix.gif files to the downloads folder.
On the webpage (pub file) place the thumbnail pixs.
Now create the hyperlinks using the thumbnail pix to the enlarged pix files,
i.e.,
Left click on the thumbnail, from the menu select Insert, Hyperlink, in the
box type,
http://www.yourdomain.com/downloads/pix.gif
The pix address in this form will allow you to access the enlarged picture
without going to the website.
If you wish it is needed to be viewing the website page to gain access to
the enlarged picture you can shorten the link to,
/downloads/pix.gif
Note: you can get Irvanview at
http://www.irfanview.com/