1. May I edit these image and then copyright them? For example, I purchase a image of a baby sleeping, can I edit the image putting a baby pillow behind the baby and copyright it?
Well you will own the (c) on the modified image, but as it is not a complete overhauling and the source image will still be an important part of your picture, the original creator still owns the (c) on his part. In other words, you will not be free to sell the image. But you could sell pillows that make people sleep like babies...
2. Using the example above, can I use this image in order to sell my products? In my website, Craigslist, eBay, Amazon, etc.
That's what stock images are for. In your example, you could use the image to sell all items related to babies (or even unrelated to babies). If you want selling mugs with the modified image on, you will need an extended license.