It is called a watermark. I'm sure googling it will give you some decent hits. Something like this: Learn Photoshop Elements 9 - Adding a watermark | Adobe TV might get you started Instead of doing text, use your own image on a second layer
Are you looking for a script / action to add your watermark to many images automatically?
Google on the word "action"...something like "actions in elements 9"
Adobe has a bunch of free tutorials, some video on their website for basics.