After spending some extra time to properly crop and resize screenshots for my Minecraft post yesterday, notch replied to another twitter follower regarding screenshots.
To take a screenshot in Minecraft, hold F1 and then press F2. Holding F1 by itself actually hides the game UI.
The Minecraft screenshots are then saved to different locations depending on which OS you’re using.
In Windows 7:
C:\Users\ *USERNAME* \AppData\Roaming\ .minecraft\screenshots\
In Mac OSX:
/Users/ *USERNAME* /Library/Application Support/minecraft/screenshots/
You may need to enable “show hidden files" in Windows 7 to see the AppData folder.
It looks as if the current size of your Minecraft window determines the dimensions of the screenshot.