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.)
See also
.gif is controversial in that there's an "intellectual property" dispute about its use. One of us will explain this at some point.