You can do it multiple ways.
From desktop/computer if you look at the tool bar above the quick reply window you will see the buttons on the second row, near the middle will allow you to post a picture or video clip.
Or you can grab the
Official AC App when replying to a post you will see the paperclip icon which will allow you to add a post.
There is also the
AC Forums App which is AC's version based off vanilla Tapatalk.
Any more questions feel free to ask.