Preview is, well, for previewing. SO, no way to actually save a slideshow, and the option to restart automatically just isn't there in preview.
There lots of other software that will give you that type of slideshow.
iPhoto will do that easily.
iMovie, and QuickTime player should do that, if you have upgraded to QuickTime Pro. Other Apple software may do that, for example Keynote - or move to some of the third party apps that give you good use of slideshows.
Go to Versiontracker, which shows a dozen or so different apps that will help you create photo slideshows.