The tool of comparing and contrasting is something that we use every single day. We compare and contrast pretty much everything from pictures to who we will vote for as our next president. Comparing and contrasting is simply the looking at the advantages and disadvantages of something and putting those advantages and disadvantages to something else. An example of this could be trying to decdide what car to drive. To decide this you would put the good and the bad things up against the good and the bad things up against the other car. 

When you use this with visuals you want to use a basis for comparison. When you are looking at two different pictures you can use this basis for comparison by finding what the two things have in comon then comparing them together. This is a very helpful tool because it helps you when you are writing or if you are just using comparing and contrasting in everyday life.