Image Handler is helpful because you load a resized image, so instead of loading a let's say 500 kB image, you're loading a small size version of maybe 30-50 kB. Now, on a product listing page, where you're loading 40 images, the difference is huge.

IH doesn't add lazy loader, but can be modified to include it. However, this is up to you how to implement it. There are also numerous jQuery plugins available that can do lazy-loading, but I don't recall seeing anything ready-made for Zen Cart. I might be wrong, tho.