Vic Gundotra, the head of social at Google, said at the annual Web 2.0 Summit that the images have been uploaded via a mixture of Picasa, Android uploads and people sharing photos via the network, reports The Daily Telegraph.
The website now has more than 40 million members, but lags far behind the 800m members claimed by Facebook, generating more than six billion photo uploads every month. Facebook's around 100bn photos stored on its servers make the site the biggest image-sharing platform in the world.
Sergey Brin, Google's co-founder, told the conference that there is enough room in the social media space for Facebook and Google+ to exist.
"I have seen multiple social networks co-exist," he said.
"Historically I have seen people use lots of services at the same time. So yes, you have to offer something of value.
"If it's identical to what someone else is offering and that network is richer - then [your product's] not going to be successful, but if you offer people something of value then I do think it will work.
"History has shown us time and time again how competitors change market share…so I don't see any particular wall or insurmountable barrier [for Google+]. I think Google+ has been amazingly successful...and there is no reason for its growth to stop."
Gundotra said that Google was in an "enviable position" of having millions of users already using and engaging with its online services.
"The incumbent [Facebook] has a huge advantage and if you play the same game then that's a hard game to win. We are playing a different game [to Facebook].
"Your friends, mother and cousins are already on Google - they use it all the time. But we have never given them a reason to express their identity and relationships before. And we are going to do that [with Google +]."
Meanwhile, Google's new cloud-based music download service is rumoured for a November launch, going head-to-head with Apple's iTunes.