changing background image

I don’t like to make changes to themes, but I like this theme and we thought it would be nice to play around and have a different background each season.

I noticed the background image is really two images, with bg.jpg the continuation of the top. I would also like to do this, but no matter what I set the dimensions to, it doesn’t work out. I tried adding padding, and that doesn’t work.

Where would I change this, and how do I get it to be fixed so the page content scrolls, but not the background?

Thanks