Galleries are unique in that they assemble thumbnail photos on a page and when one is clicked a light box is produced with the selected photo full size. Photos can be sequentially viewed, automaticaly viewed or manually advanced. The light box can be exited to revisit the collection of tnhumbnails. These developers produce Photo Gallery Stacks....
The most flexible is probably S4S with the ability to just grab a directory full of photos and thumbnails (optional) and jump into a Photo Gallery.