First, upload to something like imgur, I demonstrated with this cat..
Then click on the image (If you're on your phone, which i am, hold the image down and click view image) so it's just the image on it's own, then copy the link.
Then click in little picture image in the editing bar.
Then paste the link and press insert, and it should work :)
~All these instructions should be the same what ever image site you use, just applies to that site instead :P