Go to FB, click into photo, right click and "copy image location". This puts the URL of the picture into your clipboard.
Now go to your AP post and type the following:
< img width="200" src="" >
(without the spaces after and before the angle brackets)
Then back up to between the double-quotes and right-click > Paste.
Should see something like:
< img width="200" src="https://scontent-lax3-1.xx.fbcdn.net/v/t1.0-9/13413591_619498284890307_2788997810640030143_n.jpg?oh=36dff5eccf6d163b0d06bd8cd0881c40&oe=57C7BCAE" >