Step One:
Images hosted by flickr are the easiest option, otherwise you'll have to set up something on your server.
First, find the full urls for your images. This can be found from the "different sizes" link on your photo page. You can even have one of your images be private (right).
Hint: it does not really matter which image you specify for left and which for right, so don't sweat it.
Hint 2: the images should be about the same size, but don't sweat that either, as long as the scaling & rotation is the same.
Hint 3: if both images are in the same .jpg, that will work too; you don't need to crop them out to separate files (just be sure to choose your points correctly). Put the same url in both of these boxes: