There's two places you need to take a look at in that example.
First the property "Multi-face" needs to be set to "Yes" to get a unique image on each side of the box.
Then open the tiled background image of the box in Construct 2 and you will see that it's a strip of six 16x16 images making a 96x16 texture.
The first 16x16 is the left side, second right side, then front, back, top and down. You have to think of the front face looking in direction of the angle the box is facing.
