Develop the simple multi-image slideshow application

we are in the need of a simple multi image ( 28 square images on one 16/9 fullscreen ) slideshow, should be windows based, can also be mac based. The app will be used once to playout multiple pictures ( 800 ) taken at an event. App setup settings should be : select folder ( will be dropbox folder which has to be researched for new photos while running ) select image presentation time ( images show for 5-15 sec and then should fade to next image ). after setting up the app should be executed in full screen, exit via 'escape`. as soon as there are more than the 28 images in the folder it should be avoided to have double images shown.