Graphic Interchange format, a compact and popular way of storing images. LZW-encoded. Tk's [photo] type of [image] allows both reading and writing GIF files.

'''How to make a single-pixel GIF file:''' (but see also "[.gif color dot]" for a [pure-Tcl] version)
 % set im [image create photo -height 1 -width 1] 
 image4
 % $im put red
 % $im write im.gif -format gif
 % hexdump im.gif
 47 49 46 38 37 61 01 00 01 00 91 00 00 FF 00 00 # 0000 GIF87a..........
 FF FF FF FF FF FF FF FF FF 2C 00 00 00 00 01 00 # 0010 .........,......
 01 00 00 02 02 44 01 00 3B                      # 0020 .....D..;
Loading that file indeed shows a single red pixel.
(A "hexdump" command is described at [Dump a file in hex and ASCII].)
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See also 
   * [gifBalls] for examples how to specify base64-encoded GIF data directly.
   * [Creating image photo data from GIF files]