Add a PostImageUrl to an existing BlogPost via API

We are creating BlogPosts based on another systems data, and were trying to add a Post Image to the BlogPost. When the BlogPost is created, and we submit the PostImageUrl property with a URL pointing to a CDN Hosted public image, the BlogPost displays the image as expected. However if we are updating an already created BlogPost, the same PostImageUrl does not save to the BlogPost and no image is added to the BlogPost. Is this property not available for updating in the BlogPost API?