In composting nodes, there one called Defocus.
If you plan to use another renderer, not Blender’s internal, look in its documentation.
defocus, or for a still, just use PS/gimp to blur it a little. If it’s an animation, you use something like a garbage matte/gradient combined with a blur node to do that. I say that because there isn’t much foreground/background differentiation in that particular image for a z-depth type blur/defocus to be effective.
you can also blur based on a mask, as in http://wiki.blender.org/index.php/Manual/Compositing_Blur_the_Background and the following page to simulate what the defocus node does.
that’s what I was trying to say…thanks Papa Smurf