If you want to do a direct transfer image, the easiest way is to do a toner copy of the image at a copy shop, press it image side down into the raw clay, making sure it is pressed in all the way down with no bubbles, and then bake the clay as you would normally. After the clay is cured and cooled, you can pull up the paper and the image will be transferred to the baked clay.
If you want to seal a paper image in to the clay, you can use this technique here:
http://www.bellaonline.com/articles/art14463.asp
