You can follow the below steps:

1) Login to your EMM panel

2) Click on Admin->Account Configuration 

3) Click on Edit against Image Hostin Domain.

4) Enter the image hosting sub-domain in the text box. Please note the A record for the sub-domain should point to, i.e. the A record for img.yourdomainname should point to

Example:  Point the A record for the to


5) Click on Save