Screenshot

Screenshots are images taken in-game by pressing the F2 key (Fn+F2 for Macs). They can be found in the screenshots folder of the .minecraft directory, which is found by pressing "options" in the Minecraft launcher. Screenshots save as a PNG file with the following format: YYYY-MM-DD_HH.MM.SS.png, for example: 2011-02-12.25.05.png, which translates to February 12, 2011, 12:25:05. The screenshot will save exactly what it looks like in-game, so if you press F1 then F2, all of the GUI will be hidden in the screenshot. Screenshots that are loaded up at Minecraft Wiki should be taken with a vanilla Minecraft, "Far Render Distance", "Fancy Graphics", "Smooth Lighting" and hidden GUI. Also it should not be taken at nighttime, with other players on it and should be saved as .png.

Huge Screenshots
When holding shift while taking a screenshot, a huge screenshot will be saved as a TGA file, and the huge image will be 36,450 pixels by 17,700 pixels. With this extremely detailed image, you can zoom in quite far without losing detail. However, the image tends to be along the order of 1.8 gigabytes, meaning that image viewers will likely fail to open then file on computers with 2 gigabytes of RAM or less.

While Minecraft takes the screenshot, the data is stored directly to the hard drive rather than being kept in the RAM until finished. Minecraft tends to take a while in taking these screenshots, and can even crash during the process.