Photos based on x-rays, ultrasounds, echocardiograms, etc. can be compelling novelty purchases for patients. With the high cost of care, buying a photo would seem like a low-price addition for many insured patients and would make them feel like they're getting more for their health care dollars.
Profits from those sales could be put toward bills for indigent patients. It would help offset the impact of unpaid bills on health care facilities, potentially lowering health care costs in the long run.
Put another way: If people would buy a photo from an amusement park ride, wouldn't they also buy a photo of their own heart?