Posting pictures should be fine, because people have been sharing images for years, and we all kind of expect that. You might want to not post pictures of us engaging in activities that would get us into trouble with the retreat center, such as consumption of alcoholic beverages (not that I'm saying we did that, but it's kind of awkward for the Getaway chairpeople to be explaining to the retreat center administration that some of us like to goof around by drinking coffee out of Jack Daniels bottles). We like the place, and want to be able to continue to book it in the future!
Posting of videos, on the other hand, really needs to be cleared with each and every individual appearing in any segment to be posted. There are issues of copyrighted material, and there are also issues of performance. For instance, I for one don't think my performance of my song on Sunday night was very good, and that's NOT a version of it I want in circulation.