This answer assumes the Java version of the game is used, not the new Windows 10 version (I have not tried that yet).
If you just created a screenshot, the name of the created file will be shown in the chat stream. Simply click that name to open the screenshot in your default image viewer.
Otherwise, start the MineCraft launcher and activate the 'Edit Profile' button for your current profile. On the newly opened Profile Editor window, activate the 'Open Game Dir' button to open the game directory.
A subdirectory of that called 'screenshots' contains your screenshots.