It won't be held in a CSS file. The CSS will link to a file in a folder, probably a JPEG or PNG. You need to find the folder and save the image you want over the existing file.
Do a google search (for the blog software) asking where the bg image is. That will probably drum up a few hits and tutorials.
yep, that's what I meant, just didn't explain it very well.........If you are using Chrome then you can do the following:
Right click > Inspect element
Look for the body tag (the background should go blue when you hover of the body tag)
On the right hand side under "body" it will give you the background url![]()