Go to bp-core/images/ and overwrite mystery-man.jpg (128px x 128px) and mystery-man-50.jpg (50px x 50px) with your custom default avatar. Then go to mysite.com//wp-admin/admin.php?page=bp-general-settings and set the default avatar to the image you just uploaded, which will still be labeled Mystery Man.
I’m in the same boat, experienced in WordPress and building my first BuddyPress site. I can’t speak to all the specifics of the photo gallery but overall I’d say yes, a site like that is possible. I’d be surprised if there wasn’t a good photo gallery BP plugin out there.
BuddyPress functions should be pretty intuitive if you know WordPress, although BuddyPress is not as well documented. So it helps if you know enough PHP to understand how a function works just by reading the code.
