If your Photo Cart is suddenly getting a 500 error or only displaying a blank page then you or your hosting company has updated the PHP (programming language) to PHP 7 which Photo Cart is not compatible with (and will not be compatible with) mainly because of the removal of the my_sql functions which are replaced with my_sqli functions.
Click Slideshow in the main menu of the admin. On the left of the screen, under select which slideshow to use, set it to Always use the HTML slideshow. That will show the tiled watermark. The Flash Slideshow will only show the copyright in the middle.