Hi,
I'm not able to reproduce the issue. I've tried about 20 times with IE (different thumbnails, different products, different page reloads). It worked 100 % of the time on your website.
I'm thinking that the issue might come from the loading of the file from your server to your local computer. Maybe your internet connection is too slow ? Or you have some kind of proxy on it ?
The only thing I could recommend to try is to change the "image popup mode" option of the Display tab of the HikaShop configuration. Maybe using another popup system will help. But since I don't see the problem, it might be something on your end. Maybe you could try with another computer on another internet connection ?