Hello and welcome to BBB! I just couldn't possibly resist a Google on this one!!
If you use Skype on a Mac I found these instructions (don't know if they work and they are a couple of years old). However, some other posts I saw would seem to indicate that while you might get them to work on your machine, your contacts will have to add them as well.
I wasn't able to easily locate the file on a Windows Machine.
If this really works, you could use whatever .gif or .png you wanted. One good source for user uploaded emoticons is www.messengerfreak.com. You can download individual emoticons, but you'll have to scroll thru screens of them to find the ones you want. Supposedly, adult ones are removed, but I've not found this to be the case. The advantage of individual emoticons is the reduced likelihood of malware since you aren't downloading anything "bundled". I've never had any problems with files downloaded from this site (...for any type of emoticons ).
Good luck! I don't use Skype much, but perhaps this will head you in the right direction.