This is pretty weak. Mac users are allowed to modify their Skype emoticons.
Instructions
Right click the Skype application and choose "Package Contents." Go to "Resources/Emoticons."
Add your picture to the animated emoticon set folder or the emoticon set folder. In the animated folder you must save the file as a .gif, in the emoticon folder as .png.
Open
emoticons.plist
with the property editor.Add an entry
fpr
for your smiley. Name the object as the file name with .gif in the animated folder and .png in the regular folder. Add the strings that you want to appear as the smiley and as the name equivalents.Create a boolean and name it hidden. If you have difficulty with the programming aspect you can check the entries of the other smileys for examples.
Save and relaunch Skype.