came across this threat while looking for ways to use animated icons...

for the static ones:
[howto]

• find a nice source of PNG files
  (e.g.: http://cool-smileys.com/the-complete...ticons-smileys )
• save each (you are interested in) image to /home/opt/usr/share/icons/hicolor/48x48/hildon
• edit /usr/share/rtcom-messaging-ui/smilies/user/default.def (or create new file having the same structure there)
  e.g. i'm currently creating a default+skype.def which merges the two sets
• Conversation => settings => Smileys theme => pick your file

[/howto]

i have however to agree with pichlo about

• the need to maintain consistency w/ other platforms (PC, mac, Linux, ???) for set of smileys
• dito for codes used
  this isn't as trivial as it may seem (list a code that is used everywhere - it also requires to include all and every possible code variation for a certain smiley (!))

probably makes sense to create a .def file in /usr/share/rtcom-messaging-ui/smilies/user for each chat app

if you wish to get ride of the graphical stuff altogether, create
/usr/share/rtcom-messaging-ui/smilies/user/null.def with
nota bene: setting all the values to 0 effectively disables the "smilies" pop-up
(need to start a chat anew for this to work)

alas, conversation does not seem to support animated smileys. (trying to refer a GIF file (which is available in the directory) generates an empty in conversation