What are the Pro's/Con's of using a image on top of the background image?
If My background image is a 'blue sky' and I overlay an image of a cloud in the upper left corner, OR would it be best to make the cloud image part of the background? (At different screen resolutions I would always like to have the cloud the focal point.)

I see the biggest pro of using a separate image is that you can style it separately from the background thus keeping it the focal point of your page.