Changing background images

Ok I'm sure this has been covered somewhere. But I cannot seem to find the Background image file to change the background. I am using the black magic template. I have changed the css pages and the actual image that I think is the right image. Can someone post some kind of instruction, location or something to change the Background Image.Please.

Look in  the image map of your theme. You will have this theme on your pc, so go to there and look for the file there, ten upload a other with the same name to your server.

That template doesn't use a background image.  If you want it to use a background image, you'll have to change the background css rule.  The body CSS rules should be in either common.css or general.css in your templates css directory.

It is now:

Yup that did it. Just one other question. What size should the image be? I know that I can repeat an image but what is the rule of thumb for a basic image?

Thanks for your help btw.

This is what you need if you want a fixed background. Use a image arond 1600 x 900 pixels. 

{
background:transparent url(../images/xxxxxxxx.jpg);
background-repeat:no-repeat;
background-attachment:fixed;
}

You guys rock thank you very much.

RE:

The size in pixels really doesn't matter for repeating images.  The only thing you need to pay attention to is the file size of the image.  If the file size is too large, page loading times will suffer.

Ok one more stupid question. The background has been changed perfectly thanks to everyone's help But now where the logo area is there is a box that spans the whole site. It seems there is an image behind the log but when you branch off on the sides of that images is just a background color. How do i make this transparent or expand the image to span the width of the page. www.ampedin.com is the site if you would like to take a look. Just can't figure it out right yet. I'm lookin and reading and not coming up with much. As always thanks for all your help.