The following works on all browsers for my 200 figures, for any width percentage -- despite being illegal. Jukka said 'Use it anyway.' (The class just floats the image left or right and sets margins.) I can't imagine why this isn't the standard approach!
<img class="fl" width="66%"
src="A-Images/0.5_Saltation.jpg"
alt="Schematic models of chromosomes ..." />
Change the window width and the image scales obligingly.