Google Image Result https://media.giphy.com/media/Ij4ODdFEZ5d1C/giphy.gif Google Image Result https://i.makeagif.com/media/11-30-2014/_0m5n7.gif
Google Image Result https://68.media.tumblr.com/b72beab8e030351b9bf01b6859b47b3e/tumblr_mp272wptoc1sxgyy1o1_500.gif
Copy the HTTP address of the .gif (https://media.giphy.com/media/Ij4ODdFEZ5d1C/giphy.gif) and use the picture link in the post menu (right next to the smileys link). (If you are using Tapatalk instead, I don't know how to help you. Don't use it myself.)
