Screenshots are images taken in-game by pressing the (by default) F2 key or Fn+F2 for Macs and some other keyboards. Screenshots are automatically saved in the "screenshots" folder within the .minecraft directory.
If you are using Windows XP or higher, you can find the directory in which the screenshots are saved under by opening the "Run" application (⊞ Win+R and entering %appdata%. Then select the folder titled .minecraft, and then the folder titled screenshots. If this is a bit complicated, then you can just copy and paste, %appdata%.minecraft\screenshots. If you often access the screenshots folder, then you might want to create a shortcut.
Mac / Linux
The screenshots folder location varies by operating system. If you are using macOS or Linux, the files are saved in a different directory. Look at the table below to locate your directory.
macOS ~/Library/Application Support/minecraft/screenshots
You can also access the screenshots folder by going to "Resource Packs" in the settings menu, clicking on "Open resource pack folder", and going up one level.