This was on a domestic US flight (probably AA or United) about 5 years ago, but I took a framed movie poster (poster was signed by the cast and crew - and I had a small role in the movie!) on board once. I explained to the GA and flight attendants what it was; they were curious about it, thought it was cool, and actually let me put it behind the very last row of seats.

I may have been lucky. And on an international flight, who knows how they'd react?