OK. The image itself should be 1500 pixels wide by 750 pixels tall, for purposes of this example. Give it a black border so you can clearly see what's going on.
The fact that there's a DIV, SPAN, and another SPAN there is due to supporting code-- stuff which handles rendering markups, etc. It expects a certain structure, and I can't toss that away.
The SPAN ID'd as FRAME frames the image (duh?). I can use this or the image style itself to control the size of the image.
If you fill in the image and try printing (at least with "letter"-sized paper-- 8.5" by 11"), the resulting image aspect ration will be skewed. On screen, the image is 2:1. On paper, it's more like 2.09677:1.
If I specify 50% height for the FRAME span, then the image is stretched to 50% of the paper or window height.
It's also worth noting that I have no control over the dimensions of the paper media-- it could be letter, but it could also be legal, or A4, or whathaveyou.