First, thank you Experts! You all rock!
Here is the current code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd
ndImage = "url(green.gif)";
test <br />
<input type="button" value="Change Background Image" onclick="changeDivImage()"