A screenshot function built in to the operating system is a software component in the end of the day- and due to the proprietary nature of the operating system the developers would be able to simply remove the screenshot feature or tamper with its functioning so that one could not take pictures of the picture in question. All things software within a system that are proprietary (the proprietary viewing program, the proprietary screenshot feature/utility and the proprietary display system (whatever that may be) could work in tandem to prevent that sort of thing. The whole point of proprietary code is that the users have no control over it.
The only way one could take a screenshot would be externally, using a camera.
