I think you missunderstood what the context path is.
If you application is deployed on yourdomain.com/app, the context path will be /app.
It is used to tell the client where to look for resources.
When you do contextPath+"/images/default.png", you the path would be dependent of the deployment path (in this case it would be the file /app/images/default.png).
If you want the file next to the installation of your application server, you can use "images/default.png".
If you want to access resource files, you may want to try Thread.currentThread().getContextClassLoader().getResource("images/default.png") instead of files.
If you want to check if a context related resource exist, you can do it as stated here:
boolean exists=req.getServletContext().getResource("images/default.png")!=null;`
or
String path=req.getServletContext().getRealPath("images/default.png");`