It's kind of tricky but I found the emoji in Google Chrome under the Edit section. But you can go to an emoji website and just copy and paste the ones you find also. I don't think the mobile version can do it but I don't know because I haven't tried. Probably can copy and paste there also. (edit: the mobile can also do emoji and gifs, etc.)

Yes, Steemit has the ability to do everything including gifs, video, or anything else in your comments or posts. (use the direct link or things like that from other websites. Steemit is awesome! :)