For the past couple months I've been pondering this conundrum, I would be interested to know how many art devs consider things like this.
But I've figured out that it's possible to streamline an image into the page such that it does not get loaded as a whole.
One could even send out a dummy image, the next game I release will be using such methods, and it will be part of my open source library.
Generally bringing some of the benefits of binary compilation to html thanks to the html5 canvas.