What Dan said IS what it means.
A “pencil pusher” is something that does boring paperwork
A “pixel pusher” or someone that “pushes pixels” just means that they probably do boring work but on the digital side. Pixels are what you work with on the computer, and thus that’s what they do (instead of pencils and paper, it’s pixels and a screen).
Basically, it’s just a phrase used by those that play around in Photoshop, probably doing mundane graphic work (like color correction, batch resizing of photos, etc).