Obviously the sponsor has the final word regarding use of the content.
There's nothing much you can do about leeching if its part of a zip from the public program, except stop promoting it if someone leech you continuesly and the program owner does not care. One solution can be to watermark the licensed pics (if you get permission to do so).
"Who leeched who" is not hard to settle. If none of you have access to manipulate the external server time, you can both dex the directory and see the dates
