It would be difficult to quantify what an average person might know, but I'd argue that if you were interested in knowing what he looked like, then you'd be hard pressed not to find some illustrations or images of his likeness. If you had access to newspapers you'd have at least seen cartoons depicting him. A large percentage of the population was illiterate, and newspapers often used images and cartoons to tell a story, and Lincoln was no exception. Few outside Illinois would have known his face before 1860, but in Illinois his infamous debates with Stephen Douglas in 1858 for the Senate seat led to several cartoons and illustrations (some exaggerated, some more realistic) of the event. By his bid for president, Lincoln's face was all over the place. So much so that a letter from a supporter convinced him to grow a beard. She felt it would encourage men to vote for him if their wives thought him handsome, which a beard, or more accurately "whiskers" was sure to do in her estimation. The fact that he did grow some is indicative of the fact that clearly he felt his appearance was not only important, but that average Americans would be able to see it and recognize he looked different than his opponents. So yes, the average American that remotely kept up with the times would have seen some examples of Lincoln's face at some point or another.