Tim,
You are of course correct but... this is a very generous and commercially harmless gift/prize that hardly warrants reference to International copyright law.
To produce a poster 20"x30" you will know that the image will need to be 200-300dpi and a fair size to get a decent image. Not easy to come by unless you took it yourself, but then again some of us to tracks with our 'friends' who would never consider legal action against one another (albiet a fun ideas for the new year).
Im sure you mean well with your words of caution but making one poster is hardly hurting anyone now is it? (Unless of course it turns out to be one of your pictures
:P and then gets sold on for masses of cash
)
Seasons greetings to one and all.
Mark