SmileyImages
This option sets the list of smiley's you want to use in the "Insert a Smiley" window. You can add/remove smiley's by adding or deleting them from the list.
Example: adding a new smiley called "really_happy.gif"
FCKConfig.SmileyImages = ['regular_smile.gif','sad_smile.gif','wink_smile.gif','teeth_smile.gif','confused_smile.gif','tounge_smile.gif', 'embaressed_smile.gif','omg_smile.gif','whatchutalkingabout_smile.gif','angry_smile.gif','angel_smile.gif','shades_smile.gif', 'devil_smile.gif','cry_smile.gif','lightbulb.gif','thumbs_down.gif','thumbs_up.gif','heart.gif','broken_heart.gif','kiss.gif', 'envelope.gif','really_happy.gif'] ;
NOTE: If you want to add a smiley remember that the default size of FCKeditor smiley's is 19x19 (pixels), so your new smiley should be the same size.