Picture Slideshow

This is probably a stupid question - but: how do you control the amount of time a picture is shown in a slideshow? The number of sec's in the slideshow settings seems to control how fast pictures are switched but the actual display time of each picture is much shorter on my system. So when running a slideshow the screen is black mostly and each picture flashes on/off at the end of each display period. Only when pausing the picture comes on full time.
(XBMC-Eden, Transparency, Ubuntu 12.04, Intel HD graphics)