What you really want is a property file - for example like this http://www.easywayserver.com/blog/java-resourcebundle-properties-file-jsp/
# image default
myimage.default=../myImage.png
and
<%@ page language="java" import="java.util.*" %>
<%
ResourceBundle resource = ResourceBundle.getBundle("commonVariable");
/// commonVariable.properties file will be in WEB-INF/classess folder
String defaultImage=resource.getString("myimage.default");
.
.
.
if (imageurl==null) imageurl=defaultImage;
%>
<img class="myStyle" src="<%= imageurl %>" />
Alternative if you must
https://stackoverflow.com/a/2104580/295783
function setImageFromBG(img) {
var style = img.currentStyle || window.getComputedStyle(img, false),
img.src = style.backgroundImage.slice(4, -1);
}
<img class="myStyle" src="<%= imageurl %>" onerror="setImageFromBG(this)" />
May loop if the bgimage code fails to give a good image too