If you're familiar with the registry editor, you can run REGEDIT and browse to:
HKEY_CURRENT_USER\Control Panel\Personalization\Desktop Slideshow
Then, change the Interval DWORD to a decimal value in milliseconds (i.e. 10 seconds = 1000 milliseconds).
You may need to restart/re-login for this to take effect. Just be careful when editing the registry, as there are risks associated with it.