I did this in Photoshop so it might not be 100% exactly the same but it's most likely very similar.
Extremely idiot-proof guide incoming:
Presuming you already have your avatar ready, just save it as a .jpg
Save it wherever you want and name it whatever you want. Just make sure you pick JPEG
Now you go to your User CP
Pick 'Edit Avatar'
Choose 'Use Custom Avatar' and then browse.
And now you just search for the avatar you just made, select it, press 'open' and 'save changes'.
Now if you were talking about uploading a head texture to your tori, then that's a whole 'nother story