1 - Open a big image in fullscreen
2 - zoom in the image until it fills all the monitor (so there are no more black stripes)
3 - stay in fullscreen mode and go to next image by pressing page-down
If the next image doesn't fill all the monitor (i.e. a vertical picture), you will see the previous image in de background instead of the black stripes.

Thank you for this absolutely great piece of software!!

Matteo