Lets analyze each words first:
THINK: Use your imagination....come up with creative ideas...for this topic.
INSIDE: As in opposite of outside. The idea must lead you to interior section of the box. The picture must show or invoke curiosity about the "inside" or “contents” of the box.
THE BOX: I am defining this to be any 3-dimensional structure that can be considered to have 'inside' and 'outside'. It must have some volume. It’s not limited to any shape or size, big or small. Keep in mind that 2-dimensional structures such as circle and square are not allowed, however sphere and cubes are.
“THINK” is the key word here. This means you have to use your brain(box).
Meaning from the dictionary of the work BOX: Pack, package, sachet, envelope, bin, can, container, enclosure, bag .... all are acceptable forms of box. Also type of the boxes can be: pill-box, boom-box, mail-box, trash-box, sound-box, cereal-box, lunch-box, jewellery-box, black-box, idiot-box...etc etc. you should get the idea by now.
Its up to you how you want to interpret it.