Random _x movement with fixed values

I followed some of the tutorials about random movement, but I couldn’t find where I was looking for! I’m looking for a random movement only on the _x axis with fixed values. I will explain myself a little better. I have 10 pictures (120 x 120 pxl) with above that a transparant mask (120 x 120 pxl) I want the mask to move randomely over the pictures but so that the mask completely cover a pictures so the random movements should be a all cases a plural from 120, wich means (120, 240, 360, 480 etc)

Anybody any ideas?