I think one of the things you need to think about when designing any mod is "Does it do anything better than what is currently available around the net?". A built in gallery doesn't do this if you ask me; while yes, the administrators/mods will use the gallery to fill it with content, users wont. We had galleries in vBulletin, and I could never get my users to use them and I dont think even XenForo can change that.
They instead uploaded their pictures to flickr, photobucket, imageshack or one of the many free hosting websites out there. An image gallery ties the user into the host of the gallery, and people would rather not have their images tied to a specific forum, but to a gallery website like flickr. So honestly I find an image gallery as one of those mods that admins install because they think its "cool", but in the end, doesn't bring anything beneficial to the table.