First off, you need to go to settings > reading and set your blog to show perhaps 5 posts at most per page as your site loads really, really slowly for me on my wireless internet connection.
Go to the CSS edit window, delete anything that is right now in the CSS edit window and paste the following in there and then click save stylesheet.
background: url("http://erlhov.files.wordpress.com/2011/05/darkwood5.jpg") repeat fixed left top #0E0D0D;
When doing CSS here, what you are going to want to do is to add only the specific selectors, and the specific declarations you are adding or changing to the CSS edit window and then your additions or changes will overwrite the existing. Do NOT paste the entire stylesheet into the edit window.
The problem was that the standard CSS uses the short form background declaration as such:
background: url(URL of image) repeat scroll 0 0 #F3F3F3;
And what was entered for your background declaration was the long form stuff (as below) and it will not override the above short form declaration used in the original CSS (as above).
background-image: url(URL of image);
background-position: left top;
If you had not added any CSS at all yet, then this might be a bug.