Usually when you mouseover an image, another image shows. With that in mind, if you have five hotspots to magnify, you need five more images, one for each hotspot.
Those images would be exact except for the hotspot. You can do that in PhotoShop with the Spherize filter.
To make sure that they are all the same, make them from the same copy, and use "save as" for each one.
First select the spot that you want to magnify. Then go to Filters/Distort/Spherize. That's it.
There might be another way but for me with the programs I have, this is easiest
To make sure that the user doesn't have to wait, you should pre-load the images.