In the HTML source code, you want to look for the name of the file being used for the background. This will be contained in the BODY statement near the top. There are lots of possibilities, and I'll try to discuss the most common. This is the tricky part. Determine the directory the web page resides in.
Directories are usually one single word, without periods.
If there is no filename, leave the URL intact. Fill in the location of the image from the body statement. The path in the body statement does not begin with a slash. Simply type in the characters from the body statement, including any periods and slashes. If the path in the body statement is longer, go ahead and type the whole thing in and open the image.