You open up your favorite paint program and open up the box template texture then put the pic of whatever you want where is supposed to go like front in the front part of the template and then when you are done you save it under a different filename.
When you run Magic FPS you import the box object 2 and then click on outside texture and find the texture you made and load it.Then just scale it and save it.It might also be a good idea to hit ENTER to place the object on the floor.
My new favorite superhero,Scale Man,will halp you scale your object.