Smiley is a package separate from Gnus, but since Gnus is currently the only package that uses Smiley, it is documented here.
In short—to use Smiley in Gnus, put the following in your ~/.gnus.el file:
(setq gnus-treat-display-smileys t)
Smiley maps text smiley faces—‘:-)’, ‘8-)’, ‘:-(’ and the like—to pictures and displays those instead of the text smiley faces. The conversion is controlled by a list of regexps that matches text and maps that to file names.
The alist used is specified by the smiley-regexp-alist
variable. The first item in each element is the regexp to be matched;
the second element is the regexp match group that is to be replaced by
the picture; and the third element is the name of the file to be
displayed.
The following variables customize the appearance of the smileys:
smiley-style ¶Specifies the smiley style. Predefined smiley styles include
emoji (use emoji characters), low-color (small 13x14
pixel, three-color images), medium (more colorful images, 16x16
pixel), and grayscale (grayscale images, 14x14 pixel). The
default depends on the height of the default face.
smiley-data-directory ¶Where Smiley will look for smiley faces files. You shouldn’t set this
variable anymore. Customize smiley-style instead.
gnus-smiley-file-types ¶List of suffixes on smiley file names to try.