What image doesn't the Image Handler pop-up window show? cached large image or originally uploaded image?

Zencart: 1.5
Image Handler: 4.2

It seems that my website shows originally uploaded picture. eg.
http://www.ninesilk.com/silk-womens-...reen-p-48.html

Should I change it to show IH4 created large picture? Is it going to be faster if the pop-up window shows cached large picture? How should I do that?

Thank you very much.