You will need to setup your URLs with your DNS hosting company before following these steps.
Login to your Emercury panel and update your URL Manager. To do this go to "Preferences - URL Management" located at the top right of your panel.
Enter your img.yourdomainname in the field that says Idomain. Click Check Domain.
Enter your url.yourdomainname in the field that says Cdomain. Click Check Domain.
You will see a green check mark if everything was set up correctly.
Thats it! You're done. That wasn't so hard was it? Let us know if you have any questions.