You have an option to select background from phone photo gallery or default background from app gallery.
Note:After Installation "Gallery Keyboard" you have to chose or select "Gallery Keyboard".
Settings:Click Settings-> Language & Input (Key Board)->select "Gallery Keyboard" then click on "Default" final you can select "Gallery Keyboard".
Select Keyboard background from phone gallery by click on ''Pic photo from gallery'' or select background from below default images.
Note: After Select you have to restart phone or once change orientation for settings changes