Center Column Width Different in Different Browsers

I "measured" the center column in Konqueror and found it to be about 610 pixels. I set the maximum image width to 600 and figured all was well. I then looked at the page with image in FireFox (3.0beta) and the center column is about 510 pixels and the image overlaps the right column. More or less the same in Opera.

This is likely the same problem as in #227514: Middle part overlaps with right part!, but with a different version.

I assume this is because sizes are specified in ems, not pixels. But, the issue is how to get images to work properly in any browser.