Where are my Screenshots?

On Windows, your screenshots are stored in the %appdata%\.minecraft\screenshots folder. On Linux, they are stored in ~/.minecraft/screenshots.

To get to this folder on Windows XP/Vista/7 open the start menu, select "Run" and enter %appdata%. Navigate to .minecraft > screenshots to get to your screenshots.

To get to this folder on Windows 7/8/10 open the start menu and enter %appdata% in the search bar. Click on Roaming, navigate to .minecraft > screenshots, and there are your pictures.

To get to this folder on Ubuntu, open the file explorer (nautilus), press ^L (CTRL + L) and type in ~/.minecraft/screenshots, then press Enter ↵.

Have you looked in %appdata%\.minecraft\screenshots folder?

Taken from here: http://www.minecraftwiki.net/wiki/Screenshots

Since Minecraft 1.7.2, you can click the screenshot name in the chat to reveal the file's location.

"Saved screenshot as" messages can now be clicked to open the screenshot, takes you to the .minecraft folder

^{Source: http://minecraft.gamepedia.com/Version_history}