Why Images are Backgrounds instead of HTML Images?

I'm curious about the reasoning behind making images in ES a CSS background instead of HTML images? This makes it very difficult to manipulate them in terms of styling. I was testing out the new lightbox feature (which is totally awesome by the way). I was attempting to make a dark theme out of the lightbox and wanted to add thinks like a box-shadow and possibly light round corners around the images. But then I realized that the images themselves are produced from a background rather than HTML which makes this pretty much impossible.

I can still of course make other various edits to the lightbox which is nice.