To insert a smiley, just type in your favourite from the table below. They're only converted to an icon though if they are surrounded by whitespace.
Also, you can add any image of your theme img folder by the Icon macro.
Example
Result
<<Icon(admon-caution.png)>>
<<Icon(admon-important.png)>>
<<Icon(admon-note.png)>>
<<Icon(admon-tip.png)>>
<<Icon(admon-warning.png)>>
<<Icon(alert.png)>>
<<Icon(angry.png)>>
<<Icon(attach.png)>>
<<Icon(attention.png)>>
<<Icon(biggrin.png)>>
<<Icon(checkmark.png)>>
<<Icon(devil.png)>>
<<Icon(draft.png)>>
<<Icon(frown.png)>>
<<Icon(icon-error.png)>>
<<Icon(icon-info.png)>>
<<Icon(idea.png)>>
<<Icon(moin-attach.png)>>
<<Icon(moin-bottom.png)>>
<<Icon(moin-conflict.png)>>
<<Icon(moin-deleted.png)>>
<<Icon(moin-diff.png)>>
<<Icon(moin-edit.png)>>
<<Icon(moin-email.png)>>
<<Icon(moin-ftp.png)>>
<<Icon(moin-help.png)>>
<<Icon(moin-home.png)>>
<<Icon(moin-icon.png)>>
<<Icon(moin-info.png)>>
<<Icon(moin-inter.png)>>
<<Icon(moin-new.png)>>
<<Icon(moin-news.png)>>
<<Icon(moin-parent.png)>>
<<Icon(moin-print.png)>>
<<Icon(moin-raw.png)>>
<<Icon(moin-renamed.png)>>
<<Icon(moin-rss.png)>>
<<Icon(moin-search.png)>>
<<Icon(moin-show.png)>>
<<Icon(moin-subscribe.png)>>
<<Icon(moin-telnet.png)>>
<<Icon(moin-top.png)>>
<<Icon(moin-unsubscribe.png)>>
<<Icon(moin-updated.png)>>
<<Icon(moin-up.png)>>
<<Icon(moin-www.png)>>
<<Icon(moin-xml2.png)>>
<<Icon(moin-xml.png)>>
<<Icon(ohwell.png)>>
<<Icon(prio1.png)>>
<<Icon(prio2.png)>>
<<Icon(prio3.png)>>
<<Icon(PythonPowered.png)>>
<<Icon(redface.png)>>
<<Icon(sad.png)>>
<<Icon(smile2.png)>>
<<Icon(smile3.png)>>
<<Icon(smile4.png)>>
<<Icon(smile.png)>>
<<Icon(star_off.png)>>
<<Icon(star_on.png)>>
<<Icon(thumbs-up.png)>>
<<Icon(tired.png)>>
<<Icon(tongue.png)>>
For more information on the possible markup, see HelpOnEditing.