This is a fun approach to understanding the importance of ratios and proportions when working with visual arts: I took the famous Gilbert Stuart photo and cropped it so that it would be 2" x 3" and then made enlargements based on adding 1" to both the width and the height. The result is a distorted George that looks like he has had a tooth removed;
Seriously, the idea here is that your students look at a series of photos, measure them in inches, and then organize that information into a chart where they see for themselves what happens when you don't use multiplication/division in order to enlarge a photo.
Students write about their results and then read about how to set up a ratio between the width and height of the photo, and then compare it to other sizes to see if it forms a proportion. The follow-up activity is to look at 16 different picture frames (not drawn to scale) and then decide which ones are proportional using a scaling factor.
Comes complete with answer key and full instructions on how to teach the lesson as well as other tips.
My 5th graders really enjoyed this, especially looking at how GW changed through successive enlargements.