New York Public library has some incredible sources. I just took a PD there, and one site that really should be used by every teacher is listed below. It allows you to record vocal commentary on their thousands of documents in the digital collection. It's free, and you and your students can make comments, creating a classroom discussion about a picture or other document.
Hey, just in case anyone else is just getting around to checking out these amazing resources, the link above doesn't work. This one should be good to go for you: http://bit.ly/cLQKfi