You have to find the image you want elsewhere, hit right-click, "copy image location", then when you come to your post, see this?
click on that, paste in the image's link, and there you go.
Or in your post type (img)
http://www.example.com/werekoalaspicture.jpg(/img), except with [] instead of ().
Some sites don't allow linking, and your post will come up blank. To test that, just hit "preview post" and see if the image is visible to you or not.