image width in percent

 URL:  http://electromontis.net/evoligion/_A/A-1.shtml#A-1

On this page, and many others, I have images which are currently sized
with tags such as:

with width in percent. This means that they scale nicely on different
sized screens, and
it works on everything I have tried, keeping captions neatly beside
the images. The
validator insists that only 'px' should be used for widths, but this
gives erratic results
on displays with different pixels/cm densities.

What to do?

Ben Becula