One aspect of critically reading a visual is to help understand how it relates to the text or the author's claim.
Critically reading is the process of reading that goes beyond simply understanding the text. It involves evaluating the reading, thinking about it's strengths and weaknesses, and looking at how the reading relates to things of a similar topic. So, while looking at a visual it is important to understand how it relates to any text or the claim the author is trying to make. The image can also help you understand the tone the author is trying to convey. An image may also help the author appeal to his audience in different ways.
