It may show smileys on the whole page (so, you will have smileys everywhere) or in your Articles only (working as content plugin), or it may be used by any Joomla! extension (as an example, it is integrated into yvComment extension already).
yvSmiley ships with two ready to use "Smiley sets" (from TinyMCE and phpBB), that you may customize, and it also supports aMSN (Alvaro's Messenger) skins, so you may choose from many existing "Smiley sets" and use them without any modification.
(Read more on the Homepage...)
i think for custom.. you have to install something elsewhere beside the smiley package from those sites..
over all this is an ok plugins... too bad the custom didn't work for me but it's not important for my site anyway..