all done in Photoshop... just quickly (cos i'm typing one handed with a screaming baby in my arms):

• slice the image into depth layers- not just background / foreground but slicing the character into pieces- on mine the top layer was the front edge of the face + gun
• open Animation window in PS (if you don't know how to animate in PS look up a tutorial)
• make 7-8 frames
• in each frame, use Nudge tool to move each layer left or right (you gotta figure out which ones go where- backgroudn and foreground are moving opposite directins, with the neutral point at the mid depth of the characrter))
• i used a mathematical order, eg, first layer increased by 1 nudge (so the frames wen 1 2 3 4 nudges), next by 2 (so it went 2 4 6 8 nudges), then back again. extreme foreground + extreme background move the most (edit: this is where i fucked it up though, i should just do that stuff by sight, and make the increments smaller)
• fix up edges that don't work (clone stamp tool works well for this)

sorry i can't be more detailed. baby screams are kind of distracting