Hi Rishav,

As per the instructions mentioned by IIM Calcutta, the conducting body for CAT 2018, the photograph should not be more than six months old and should have a White background. If your application form has been submitted with non-white background, you may get your admit card. Also, an application correction window is usually given by IIMs where candidates can change certain details in the application form. This window might open in the fourth week of September. 

However, I would suggest you contact the admission authority through email and mention the issue. Here are the contact details -

For more updates on CAT Registration and CAT preparation, visit the hyperlinks.

All the best.