How do companies like Best Buy optimize their email images so well? Big, one column image one after another as you scroll with fast download. The last email I received I counted at least 10 product images instantly appearing on my mobile phone.

How do they do that? Other then the conventional image optimization in photo editing programs, there's got to be some secret ingredient to their fast speed? A faster, more powerful image server, a js image loading script, or a combination of various methods?

Anyone have any insights?