it's actually rather simple. when you go to create a meme, in the template section there should be something that says "Upload template". it will ask you to choose an image (most likely from your computer), upload the image you want, and then it will take you to a section where you add the name, aliases, etc. once you've done that, click finished, ok, upload, whatever the button's name is, and the template is uploaded. NOTE: i'm not sure if they are public by default or not, you'll have to check that yourself. hope this explanation helps.
