The following are the steps to replace/change MDM login screen background image.
1). Stop MDM application server.
2). Replace the existing back_login.jpg image in ECM.ear/EML.war/images with your image.
3). Make sure it's dimensions are in proportion with the existing (default) back_login.jpg i.e 1920 * 427
4). Restart MDM. Clear browser cache if the changes are not reflected.