What you will need to do is go to your "style.css" page under Appearance then Editor.
Look for this:
background: url(images/bgr.gif) #B8CFEB repeat-y center;
font: .8em Arial,Tahoma,Verdana,Helvetica,Sans-Serif
Upload your image file directly under the images folder. If this doesn't work then try going to WP-CONTENT >>> THEMES >>> WHATEVER YOU THEME IS CALLED >>> STYLES >>> YOUR THEME NAME.CSS. You will then want to look into the .css file that you are currently running and look for something like "bgr" or just the words "background".
Let me know if this helps. If not, I'll be sure to follow through to help you get it right.