Yahoo has acquired the startup Luminate, a service that focuses on advertising on top of online images, as the Internet giant tries to give its once-mighty ad business a jolt. The service shut down earlier this week, and terms of the deal were not disclosed. TechCrunch earlier on Friday reported the news. The company, founded in 2008 in Mountain View, Calif., is an ad network with more than 180 million users and more than 6 billion image views each month, according to Luminate’s website. The service lets marketers layer product, display and text ads over images.