Add Images to Product:
Click on Products and select Products
Select the product for which image has to updated. Use the filters for quick access to the required product
Click on Upload Image. Browse and select the image that has to be uploaded. Click on Save upon successful upload of the image
Image size should not exceed 2 MB
Sync changes to Zomato
Click on Settings and select Integrations. 
Click on Online Orders and select the integrated delivery partner for which images has to be synced
Click on Sync Menu and wait for few seconds for the successful upload of the changes
Refresh the screen after 60 seconds. Refer to the "Sync Message" to know the status. Following message will confirm you that the latest menu is updated "menu_sync_update_callback_message: Menu successfully updated"