I have (re)installed zen so I am talking about a completely original version without any adaptions. I added three products, installed the image handler and everything works fine.

But...: I would like to increase the size of the images that appear when you hover over the smal picture in the product listing. I thought I could do so by increasing the size of the medium pictures

So I changed the value for both
"Product Info - Image Width" and
"Product Info - Image Heigth."

If I do so, eveything in Firefox still works fine with indeed larger images, but in Internet Explorer I get an error on the page and the hovering images are not shown anymore.

Can somebody please tell me what I am doing wrong?