This is based on the photo’s EXIF data which lists its resolution as being 1944×2592, suggesting a 5MP sensor. It was also revealed that the front-facing camera would also have an aperture of f/2.0, making it a relatively wide and also good for low-light photography. Other details have also revealed that the photo was taken inside Googleplex in Mountain View, California.
Given that the Nexus is usually commissioned by Google, there is a good chance that a working prototype has already been made and is being tested by Google, which might explain why the device was at Google’s headquarters. That being said, there is always the possibility that the EXIF data was faked.
This can be done but at the same time with the end of the year creeping up on us, we wouldn’t be surprised if Google and their partner OEMs have been busy putting the device together for a possible late 2015 launch. Either way take it with a grain of salt but we’ll keep our eyes peeled for more info.