I've used the free version of 2Flyer (older version is the only one that's free, I think) and it works pretty well. Decent range of options, uses images, can set music, transitions, etc. It doesn't do animation as far as I know, so it may not be able to do what you want.
I think first explaining the kind of screensaver you want to make - the requirements of the actual "show" - would be good so people can make better recommendations as there are really a ton of these kinds of apps. Are you going to include animation or just stills? Audio? Video? Why is multi-monitor support required, and do you want it to just duplicate onto the 2nd monitor, stretch, or just blank it out? What other features might be needed besides those mentioned?