How do I superimpose an image in photoshop?

I have a special request from a customer and am not too savvy in Photoshop to know how to do it. She gave me a 3-year-old picture of her mother and asked that I put it in the family picture from this year. I want to make it subtle and have told her my ideas, AND that I have never done it before. The family includes grandkids, cousins, the whole shabang...someone please walk me through this. The pic has been scanned in as a PDF, and the family pic is a jpg. I have Photoshop Elements 7.0.