To add a fallback image:
On the left panel, click the Settings tab.
On the Settings section, click Select Image. Canvas opens the assets tab.
Select a fallback image from the images you have uploaded and click OK. The fallback image is selected.
In the Landing URL section, enter the landing page URL. In the rarest case where your creative is not rendered, when users click the fallback image, they will be directed on the URL mentioned here.