This is how it can be done:
Step 1: Log into your CG account
Step 2: Click on Manage Tests on the Menu bar
Step 3: Select Test template and click on Edit next to the concerned test
Step 4: Choose the concerned Section from the Side bar
Step 5: Click + Add MCQ and select Image icon from the Format bar.
Disclaimer: The images can be added only in the question section. You can add questions directly into the library AS WELL.