Sixel is a terminal graphics format from DEC.  It allows graphics to be stored
and transmitted as text using terminal control codes to display an image.  It
is supported by serveral terminal emulators including XTerm.

Sixel at Gopherpedia

Learn about how to get Sixel graphics

libsixel(Github)

If you want to deep dive, The Lambda Lab has a great technical document about
sixels.

All about SIXELS

Most gopher clients won't handle sixel graphics correctly even if your terminal
supports it.  Clients that use the UNIX tool `cat` will work.  `less` with the
-r parameter will work as well except it won't get the screen hight correct.

* vf-1 works ok.  The default text/plain handler is cat.  If you use the 'less'
command, it will pass -R instead of -r to the system less and it will ignore
control characters.
* sacc does not interpret the control characters.
* clic thinks the image is one line high and scrolls the content too far up the
buffer.
* lynx does not interpret the control characters.

I use vf-1 modified to pass '-r' to less.

If you can see a row of colors below, your sixel graphics are working!