Chartboost Required Images

Frame (the border around the main picture)
1836x1080
1088x640
1920x1200
1280x800

Close Button 
60x60
30x30

Main Image (this is the image that will pop up in other games that people can click in order to play our game so it is supposed to be something that people will be interested in that shows off our game)
1434x810
850x480
1560x750
1040x500
 
I don't know if all of these sizes are required. Probably best to ask josh which one's he specifically wants.