There are 6 sides of a Rubik Cube, so you can offer 6 photos to customize each side.

The photos could be same or different. 

Anyway, you need to connect with your agent, send photos to him, confirm any information you want and tell your requirement to him.

Pocket cube(second-order) 

Size:5*5*5

Weight:67g

Magic cube(third-order)

Size:5.7*5.7*5.7

Weight:96g