I checked out the page you linked to, and that product page is not using image handler to resize the image - the image on the page is still the image in the images directory - and it is quite large. The same is true on your product list pages.

The zoom function works for me, but the page that comes up shows the image at 150x75 because the image tag has that width and height set for the image.

Did you look through the "help" information on the ImageHandler4 admin page under the tools menu in your admin? This may have some helpful information. I suspect that some file (or files) on the public pages isn't installed correctly.