I'm not sure what my idea was here, beyond noting the cultural trend set by Wrox books featuring their nerdy authors on the cover. (My favorite is the duo on the Javascript Objects cover, top center.)
At first I thought: That's nice... they want to make the authors celebrities, to help improve their social lives.
But the covers probably communicate "trust" to the target audience: "See, I am just like you -- a technocrat responsible for the software infrastructure of the world -- and I take my job so seriously that I have no time for vanity!"
(I am of course a nerd myself, and jealous that no book company has seen fit to put me on a cover. If and when they do, I hope they use this picture... )