I do not believe that it is possible to do a Post with multiple photos using the Graph API. Creating a Facebook Post means sending a POST to the endpoint https://graph.facebook.com/me/feed
. In that endpoint, you can include a "picture" property in the query string. This is the URL of the picture you want to include in the Facebook post. In your created post Facebook does not reference your picture from that URL, it makes a copy.
Example POST request:
https://graph.facebook.com/me/feed?message=check%20out%20this%20picture&picture=http%3A%2F%2Fwww.mydomain.com%2Fmyphoto.png
On the desktop and mobile apps, you can add as many photos as you like but they are still uploaded individually. Using the console you can see that they are uploaded using the endpoint https://upload.facebook.com/ajax/composerx/attachment/media/saveunpublished It is only after you press the "Post" button are they then associated with your status message.