Those poster hangers with the string are good, and warios suggestion is what I was gonna say. use big ones of these,
<img src="http://www.trombonelessons.com/paperclip.JPG">
and you might be able to get the S hooks through them.
Ahhh, S hooks... Brings back memories... Good times, Good times.
Edit: or better yet, use these, and tie little bits of string through them and then loop the string over the S hooks.