I own a company that does this, and most advertisers do prefer to put an ad image instead of a QR code. We focus primarily on sites near flight paths at major airports and the Google Earth/Maps aspect is more of just an added bonus for us so there is some difference in target audience but our clients resoundingly prefer actual ads to QR codes. We've tried selling the idea of rooftop QR codes because they are so much easier to paint than e.g. a photorealistic person who is 250ft tall but haven't had any interest from clients so far.