You can replace the default Smilies that come with the WordPress installation.
[recommended]You may like to read: List of Smilies Available In WordPress Default Installation[/recommended]
The images for the smilies are present in /wp-includes/images/smilies directory of your WordPress root installation. The file names are like icon_smile.gif, icon_cry.gif and so on. Just check if these images for the respective emoticon exist in the folder.
Take the backup of this folder and then you can replace any of the image graphics specific to your Smiley with your own image with a FTP client. Note the names of each smiley file. Your files must match these names and should be in the same ‘gif’ image format. You should keep the image sizes same i.e. 15X15
WordPress Plugin to customize the Smilies
There a WordPress plugin named Custom Smilies which can be used to customize the default Smilies that come with the WordPress installation as well as using this plugin you can insert the smilies in post and comments with a single mouse click. You can also change the text that can be entered for a particular smiley face.
Thus, in case you use this plugin, you can give your commentators more expressions as they do not have to remember the text for each smiley. Just they have to click the icon of the smiley to insert it in the text. Same is the case with your blog authors