I have been wondering earlier why I cannot use themes in "Photos" screensaver on my Dell M65 running Windows Vista (I used that for a long time on my previous Acer Ferrari 4000). Of course from the help I found out that themes require at least 3,0 in the graphics performance (I had 2,3).
I just today realized that I have updated the graphics driver some time ago (it is an Nvidia Quadro FX 350M with 256MB memory), and it might make a difference to just refresh the performance numbers. And yes, after some measurements my performance now shows 3,5 and themes are now selectable in the Photos screensaver settings!
A while ago I already installed a similar screen saver that almost did the same thing, PixPile by scorbs. It is made with WPF.