The number of photos you can use in a screensaver is limited, if you have reached that limit it won't matter how many new photos you add to the album you use for the screensaver. If you want to change the photos you use in a screensaver, change the album it uses.
Kinda on the same topic, I'm building up a few screensavers for my appletv and was trying to find what the maximum number of images is. I did a quick search but haven't been able to comeup with a answer. Any thoughts or suggestions would be appreciated.