This applies to Social Feed, Announcements and Crowd Pics pages. They are the same page type, and the layout is one of the settings that differs between them.
The two layouts
List — posts appear one after another, newest first, with their text, photo or video and any comments. This reads like a conversation.
Grid — photos and video thumbnails are tiled, and tapping one opens the post. This reads like a photo gallery.
The posts are the same either way. You are only changing how they are shown, so you can switch back and forth without losing anything.
Changing it
Click the gear icon next to the page in the Editor's page list and choose the layout. Publish your app afterwards, so the change reaches people who already have it.
Which pages start out as what
Crowd Pics starts as a grid.
Social Feed and Announcements start as a list.
Building a photo wall
If the point of the page is sharing photos — a party, a conference, a trip — grid layout with comments turned off reads best. That is exactly what the Crowd Pics starting point gives you, and you can get there from any social feed page by changing those two settings.
Videos in both layouts
A video post shows a thumbnail you tap to play, in a list and in a grid alike. See video on social feed pages.