only load 12 images at a time to improve performance with lots of photos

i have a owncloud installation with over 7000 photos and displaying all of them at once was a performance problem. now with every click of "load more" 12 more thumbnails are displayed (with my resolution that is approximately one screen, maybe the number could be higher to save a lot of clicking)

i hope my code is also usefull to you and other users.

br tuempl