Re: BP Gallery/Media – Which plugin do people recommend?
1. BP Gallery generates own tables in database in addition to interacting with other BP- and user- tables. Beyond that and for more specifics, you gotta ask @sbrajesh at Buddydev.com 
Users can post images, audio and videos uploaded via BP Gallery in e.g. main site into respective blogs within a multisite installation using BP Gallery shortcodes.
2. “if a user wants to add an image that would be associated with a post that was created by the admin“
Not exactly sure about what you meant. If Admin created a post, no other user role can edit that post unless it was a Super/Site Admin or a co-Admin of the site.
