Before starting the game, just drag/drop the pictures you want to see on the big screen in the game right here! JPG and PNG are supported, and it's expecting a 16x9 aspect ratio. 

There's a list of common 16 x 9 resolutions here on wikipedia if you want to resize/crop pictures to put them in your world. The bigger the better! 
https://en.wikipedia.org/wiki/16:9