When using Desktop Slideshow in Windows 7 (a slideshow of desktop backgrounds), you can change the duration of the transition between two images with a registry hack.

1. Navigate to the key HKEY_CURRENT_USER\Control Panel\Personalization\Desktop Slideshow
2. Create a new DWORD (32-bit) value, named AnimationDuration. The value of this DWORD is set in milliseconds. Setting it to 2000 will produce a transition of 2 seconds, and so on.