You're too late to see the exhibit Picasso's Guitars 1912-1914. It ran at the Museum of Modern Art from February 13th to June 6th. Fortunately it's not too late to see what we missed. MoMA has a wonderful resource up on their site with dozens and dozens of Picasso guitars that were made in Spain and in France between 1912-1914. According to the blurb, this series is where cubism begins. Here Picasso begins pushing the still life objects closer and closer to the picture plane until he the image seemingly pops out in the form of the cardboard guitar.
The site is a fabulous resource for students (and adults). The images are a great compliment to all the wonderful Picasso lessons on the net, and especially ones that examine this theme. Lots of great information accompany the rollover images including background, materials, and techniques. Check it out. Have a lovely day!