Looping slideshow / duration control on Imac Preview

I am wanting to put together a slideshow with photography and captions (I am laying each slide out using photoshop).

There will be four slideshows to run on four new imacs. I would like the slideshows to run by themselves and loop once they've got to the end. I would like each slide to last for about 6 seconds (would be great if timings this could vary from slide to slide though as there are differing amounts of text.)

I have saved the jpg image layouts in Photoshop so that they are the ratio (16:10) of the screen so that not bits are cropped off/there is no black edges.

The slideshow in preview looks great but I con't get it to loop.
I know that a continous slideshow/slide duration controls in Powerpoint is possible but I can't seem to get the image in Powerpoint to fit the screen.

A slightly long-winded explanation I'm sure, but any help/guidance with this little dilema would be much appreciated!