Generative content creation has significant potential to transform advertising. This research examines how generative AI–enabled visual ad creation affects real-world advertising effectiveness, drawing on more than 16 billion ad impressions and 116 million clicks from a leading online ad platform. A quasi-experimental design compares 4,633 “sibling ads”—AI-generated and human-made visuals launched by the same advertisers under identical campaign conditions. AI-generated images deliver higher clickthrough rates than human-made images, but only when they do not appear AI-generated. Visual features shaping perceptions of AI generation are identified: while clearer images and larger faces are associated with human-made ads, aesthetics and intense color saturation tend to signal AI use. These findings inform both advertising platforms offering AI-powered creative tools and advertisers adopting them.