When you open a gallery, you see all the photos in thumbnails. On this page, whenever possible, you have a button "buy the whole gallery" by clicking on the price proposal.
When you click on an image, it expands and you start a slideshow. You can switch to full screen mode at any time. In this configuration, at the top right of the current photo, or at the bottom left, you have the info "i", then the "buy" button, then the "share" button and finally the on / pause button of the slideshow .