DISCLAIMER: This code was originally written for Windows 8 and a number of things have changed since then. Expect this to need some minor tweaks for Windows 8.1 and beyond.
Learn how to use DirectXTex to take a screenshot of a DirectX rendered scene.
See blog post here - http://blog.pnapieralski.com/windows-8/taking-a-screenshot-of-your-directx-game-using-directxtex/