To expand on Marc’s answer I dug into the source code of the page.
div tag in the html like such:
<div id="runner"> <img class="parallax-layer" src="http://mortenstrid.no/wp-content/themes/mortenstrid/images/runner_layer_bottom.png" alt="" style="width:1347px; height:642px;"/> <img class="parallax-layer" src="http://mortenstrid.no/wp-content/themes/mortenstrid/images/runner_layer_middle.png" alt="" style="width:1307px; height:623px;"/> <img class="parallax-layer" src="http://mortenstrid.no/wp-content/themes/mortenstrid/images/runner_layer_top.png" alt="" style="width:1266px; height:603px;"/> </div>`
That means there’s a script, or numerous scripts that interact to move the images around the page. Off the top of my head I’d say, as I mentioned in my comment, that the images rely on the cursor’s X and Y coordinates and adjust themselves according to the coordinates.
At the top of the source code we find a number of different scripts. One of them is called “jparallax” which traces back to this website, where you can find numerous demos.