It's actually pretty simple. You need a program that lets you edit plist files. I use "Property List Editor", which is part of the developer tools that come with every Mac (on the install discs).
ctrl-click (or right-click) on the Skype icon, select "Show package contents" and go to Contents->Resources->Emoticons->Animated Emoticons.SkypeEmoticonset
Here, open the file called Emoticons.plist By checking each of the items, you'll see that some have a "Hidden" attribute set. Either set this to "no" or simply delete it, and you'll see the hidden Emoticons, once you restart Skype.
Have fun