Checkpoint 3.7

I know this is extremely late in the process, but I just realized the
implications of 3.7, and I do not think it is realistic
I agree that for most elements, proportional spacing is preferable. But for
images? I thought it was preferable to set up a size so that the browser
knows how to set-up the page.
I am going to be asked to defend this checkpoint, and, right now, I do not
know how.
What am I not getting?
David Clark
CAST, Inc.