Here are some of the side by side (sbs) images I created for my students. I check my students work from the outline to the finished drawing. I edit a drawing and point out the areas and then create an image that shows a "sbs" of the reference photo, edited drawing and unedited drawing.  I find that giving visual help (like below) along with the written help have worked great!

Click a thumbnail for the large image (more sbs's are now in each image)