Dropbox Replay is an amazing tool for sharing, reviewing and giving feedback on media files, however when using still images, it is not possible to click a left or right arrow to scroll through the rest of the files within the same folder. This inhibits the ability to quickly review and comment/approve or mark-up image after image, you need to navigate out of the image back to the folder, into the next image, back out etc.
Standard Dropbox has this functionality so I would think it would be a very useful and hopefully easy (and popular) tool to implement.