LOL :P you can change the colour easily but you cant change it to a pic yet
here install take ownership from somewhere on the internet and then take ownership of the mosaic.config file and then press edit on right click
then around the bottom of the txt file there should be bacground color
go to this website http://html-color-codes.info/ and pick the colour you want
copy the text below the colour into the txt file instead of the colour text alredy there and you're done!