here is super simple to use and admin gallery app. There are a couple ways we could design the structure. One way is to have individual links that goto individual galleries. 2nd way is to create one link the software will have folders in them of galleries. The first way gives a little more flexibility. There are different "skins" we can play with as well. The default one I kinda just like.
Its pretty slick too from a techy side. The app when we go to post say a new gallery auto re-sizes the large and thumbnails. So no having to alter the originals and it creates separate copies. Thumbnails range 3-4k and the full size are about average of 75k. The test site I setup above has the large images set to 75% compressed and 800x800. They were reduced from 1.7mb @ 3000x2000.