your background must be available in the www folder of the webserver. You have two possible options for how to upload your picture to the webserver.
Copy your background to the
hmisrcfolder in your project. Then point your CSS to
The disadvantage of this approach is that you will upload your background every time you compile the project.
Use WinSCP or a similar tool to upload your background to the target. The location must be in the webserver root or below. The root of the webserver is in
/rex/wwwfolder. So upload your background to '/rex/www/bacground.jpg' and change your CSS to