Go into the settings for your photo screen saver and make sure the "shuffle photos" box is checked.
I use the screensaver that comes with Windows. I have it set to a folder with lots of photos, but it always starts with the same ones when it has thousands to choose from. What are some better screensaver programs? A search will pull up lots to choose from, but I am not familiar with them, plus I don't know how safe all these sites are.