Version: (using KDE 4.2.0)
Installed from: Debian testing/unstable Packages
Images from Compositing seem to stay in the graphic card RAM. If you logout and login again, and click on the splash-screen (so the splash goes away), you can see a mosaic of images and other screen parts from the previous session. It seems there is not possiblity to clear the Video-RAM to prevent this. This may be a privacy issue on shared systems, and its just unbeautiful for all others.
I'm using nvidia nonfree-driver NVIDIA-Linux-x86_64-180.22-pkg2.run
What you are experiencing is a distribution-specific patch bug. Further details are available in bug 170462.
*** This bug has been marked as a duplicate of bug 170462 ***