I know this is an old post but I was having the same issue and I resolved it. Since there wasn't a valid answer to this post and it came up in many of my searches in Google I thought I would add the answer.
The main issue that is happening here is a permissions issue with the App and Access Token created in https://developers.facebook.com.
You can both GET
and POST
attachements on a wall but you have to give the Users Access Token
the permission to access it in the first place. You can do it like this.
1. Go to your application on FB

2. Click on Graph API Explore

3. Click on Get User Access Token

4. Click on user_photos and user_videos
There are many options here but to get the basic attachements which tend to be photos and video you can just select those two, but you can add as many as you want.
After clicking Get Access Token at the bottom to save it you have to login with your password then give your access toke permission for your account. Once you have done that you now how permission to access your feed attachements (photos and videos)
Here is what the Graph API output looks like showing the picture from the feed
Now keep this in mind this is a Short Lived Access Token which lasts about an hour and it will stop working. You have to convert this token to a permanent token and let me save you the time you will bang your head trying to figure that out because I wrote a php function that will generate your permanent token from your short lived token all in one shot for v2.9
Check out my post reply here
facebook: permanent Page Access Token?