add a background image

[rensing]

I would like to add a background image to my site so it shows on the main page.
I would like that image be like it's just visible and in the background as to not to interfere with the main images that I show for my products.
a) can this be done? If so, how? Even if I can only have it show in the header.
b) does the image itself have to have the effect applied to it or is there a setting I can use to adjust the effect?

[dbltoe]

Several good answers below in the similar threads.