There's no functionality for that. Best I can suggest is substitute it into a Custom HTML field then IF substitution against it. Example as follows.
Code:
[cb:if FIELD_NAME!=""][cb:userfield field="FIELD_NAME" /][cb:else]FALLBACK_IMAGE_HERE[/cb:else][/cb:if]
Replace FIELD_NAME with the name of your plans image field and replace FALLBACK_IMAGE_HERE with your fallback image HTML.