Insert Image by percent rather than fixed pixels

I recently came across Joel on Software's newly redone web site and found a feature I'd like to implement. That is dynamically resizing pictures in the text column. The pictures are set to be 50% of the column width. The images maintain their correct aspect ratio as the browser window width, and therefore columns, are resized.

Like Joel's web site, I too am using a three column liquid layout from Mathew James Taylor and would like the ability to use a percentage of the column width for image width. Is this possible with FCKeditor? What needs to be done to achieve it?