It's always worthwhile to check and see if any software library (commercial, or open-source) has a support forum, and do a search there for the topic you are interested in.
In about 90 seconds, I found this:
"All you have to do is:
1. Add the icons into the table in /jscripts/tiny_mce/plugins/emotions/emotions.htm
2. Add the file into the /jscripts/tiny_mce/plugins/emotions/img directory"
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