Does the width modifier work the same way? And does it have to be an integer number of pixels, or can you use the % modifier?
No. The key to thread efficiency is height. Browsers do not care about width, they only want to place things correctly, vertically. So they will see a height modifier, put a placeholder img block there, and go on. Then the pic will load at its own leisure.
And go with pixels, not %, or more calcs are required. A simple 'height=400' gives you the best thread efficiency.
Source: Laz, the web developer.