Tom Bober and Brianna Zavadil White write: "You may have seen a portrait of a famous individual used alongside a title slide of a presentation or accompanying a list of facts about that person. In classrooms, portraits are often used as window dressing to history, a face to put with a name, event, or date, but portraits can tell students much more.
The strategy of reading portraiture encourages the visual analysis of a piece of art, similar to closely reading a document. The visual clues found in portraiture may be decoded to learn about the individual featured in the artwork. To get started, select visually complex images that include objects and a compelling setting."
Via Mary Reilley Clark, Lynnette Van Dyke, Jim Lerman
Starting students off with primary source documents can be a challenge, as they struggle with archaic language, print quality, etc. Using images, in this case portraits, and having students examine them in a variety of ways, can make primary sources more appealing! I love the suggestion in the comments to compare two portraits to really get students thinking.