Search engines such as Google and Bing are monetized through advertisements — which means that the number of eyeballs that view the results is very important. With nearly 15 percent of phones worldwide using iOS, according to IDC, Apple's default search engines represent a sizable audience, even if those users don't always click through to the respective search pages.
Siri's search powers have reportedly oscillated between Google and Bing through the years. TechCrunch previously reported Siri's deal with Google and noted that Bing will still be used to display Siri searches for images.
Analysts estimate that Google already pays billions to be the default search engine for iPhones and iPads. Investment firm Bernstein has put the figure at $3 billion, even without Siri, after legal documents revealed that Google paid $1 billion to stay on the iPhone in previous years.
Apple's software and services, including Siri, continue to become a bigger part of Apple's money-making strategy, as voice becomes a more popular interface for wearables and home speakers. Microsoft, too, has looked to make allies in this arena: Microsoft's Cortana and Amazon's Alexa can now work together.
Apple and Google declined to comment to CNBC on the details of a deal.
A Microsoft spokesperson told CNBC that in addition to Siri image search, Bing still powers the aspects of search on Yahoo (Verizon), AOL and Amazon, as well as the multi-lingual abilities of Twitter.
"We value our relationship with Apple and look forward to continuing to partner with them in many ways, including on Bing Image Search in Siri, to provide the best experience possible for our customers," the Microsoft spokesperson said. "As we move forward, given our work to advance the field of AI, we're confident that Bing will be at the forefront of providing a more intelligent search experience for our customers and partners."