This post will be a short one. While playing around with iMessage today, I noticed that you can send animated GIFs and they will animate within iMessages both for you and for the recipient. One item of note, however, is that the image will not animate within Photos.app, which seems to include clicking the message to show the image full-screen.
In case you are wondering what source you may use for GIFs, I’ve found that browsing to a website with animated GIFs on Safari works. Simply press-and-hold on the image, and select “Copy” from the resulting menu. Then, in Messages, tap in the text area and choose “Paste”.