Two ways to do that:
- Using HTML (find the picture you want to add and type [img src="paste your link here"], replace the [] with <>)
- Easier way: Find the link of the picture of gif that you want, and press Ctrl+G while typing an asnwer. It will ask you to paste the link of the picture or animated gif.
This is what will pop up:
I use the first method, don't know why, just used to it. Second one is probably easier and faster.