Talk Blogging

I use the misty look theme in wordpress. I want to add a background image to the background. One inside the box where all the contents are and one outside. How do I edit the stylesheets and such? I'm not very good with codes and stuff, so a clear step by step direction of where to put the image and the code would be greatly appreciated. Thanks.
I don't want to seem like I'm bribing... but best answer to whoever helps me get the result I want.