Rich text fields are a little wonky. ISNULL and ISBLANK don't work
with this field type, period.
To check NULL on this data type field we need to use LEN. It basically calculates the number of characters.
So if you have VR like this: LEN(richText__c) = 0
If you insert an image in the field, the rule will not fire. It will only fire once you add a space ...