it's pretty easy once you figure it out, you simply have to use the url of a pic that's already been uploaded.

For example you add something like this
h t t p s : / / s4.postimg.org/3lct62afh/image.jpg

do it without the spaces and you will link to your pic

For more I recommend a great video by @georgedonnelly "Everything you Need to Get Started Fast with Steemit, Social Media that Pays"

And that's it :) I had the same questions, good luck.