remember that there is no medium image in IH, you just upload one default image (the largest one), IH does all the resizing.
I have set all images to 80 x 80, except product info, that is set to 100 x 100 and zooms (or hovers), if you click the large (default) image appears.
You can check my site for the effect, but no criticisms please, as my sig says, I'm working on it.