Friendly Atheist compares some of the current best-selling Christian books with some of the current best-selling atheist books and notices a difference. The Christian books have a smiling picture of the author (with their hand folded) but the cover of the atheist books contain just words, no imagery. This leads him to conclude the following:

Why are the covers of atheist books so different from the Christian counterparts?
One explanation might be that the Christian books are driven by personalities. The authors/pastors themselves are the selling points of their books, moreso than their content which is essentially interchangeable.
Atheist books, on the other hand, are less personality-driven and more content-oriented. Because of that, we seem to focus the covers more on the words and less on the imagery.

(via)
I would agree that some of the Christian market is personality-driven but that percentage is small when you compare it to the larger market. And when you do, you find the following bestsellers (from Amazon):



None have a picture of the author on the cover, smiling or otherwise. Yes, they do use imagery but you would have to agree that each is selling more on content than image.
But that’s not even the extent of the market, lets take a look at some of the bestsellers throughout church history:



I’m just skimming the surface here. And I haven’t even included the best-selling book of all time:

I don’t think you can get plainer than that! (What? No smiling Jesus?)
Maybe we use imagery in our books because we understand that we don’t have to disconnect the visual from the intellectual, that the visual can convey something to the reader that author would like to invoke. I contend that the atheist authors are conveying what we would have if we accepted their message: life in a sterile, colorless world. Bound by nature and never to transcend into the supernatural.
Why no smiling atheists on the cover of books? I would think that it would be hard to smile when you say, “This is all there is. You live, you die, you don’t get anything else.” Nilism doesn’t tend to make people very chipper. (Though Friendly Atheist looks pretty chipper). See how disconcerting it would be to put a smiling atheist on the cover of a book entitled “God is Dead And How to Keep Him That Way.”
We smile because we have good news, “This isn’t it! The madness, despair, injustice of this life is not the extent of our existence. There will be a time when our tears will be dried and justice will prevail. When we will live in peace and joy.” That tends to make people happy.