I could have sworn there was a sticky about this at one time or another. I know you have to host your image on a third party site, such as Imgur, but then is it just a matter of copying and pasting the link to the image?
Off the top of my head: Pretty much, but you have to have a direct link to the image, and it has to be on a line by itself.
A direct link would be a URL that ends in .jpg or something like that. On a Windows computer, it usually works to right-click on an image and select “Open image in new tab” if the image is part of a web page.