You can't. You will have to upload it to a file hosting site, or if you have an art thread, you can upload it there and then use that link (at least I think that works).
Terraria Community Forums
Kazzymodus
SolarFlareWarrior
Uh.. I tried Imgur. I'm on mobile... halp?
darthmorf
AFAIK imgur works on mobile.
(http://i.imgur.com/5hAWqNP.jpg)
(http://i.imgur.com/5hAWqNP.jpg)