.png

Image format that uses indexed colors and lossless compression (like a .GIF file), but without copyright limitations; cannot be animated like a GIF image, though the related .MNG format can.

PNG images may also include an 8-bit transparency channel, which allows the colors in the image to fade from opaque to transparent; GIF images only support fully opaque or fully transparent pixels; PNG images are now supported by most Web browsers.

Mac OS X 10.4 and later saves screenshots as PNG files.