He released this photo because all the gun nuts were saying Obama had no right to talk about gun control because he had never fired a gun. This photo is just a way of shutting them up.
What I find most ridiculous about this whole debate is that Americans seem incapable of sitting down and having a proper discussion about the issue at hand. Regardless of whether you love or hate guns, you should be mature enough to have a reasoned debate on gun control and get your point across in a coherent manner while also taking on board other people's points of view. Not sticking your fingers in your ears and yelling that everyone else is wrong as Americans seem to love doing.