I have tried to change registry setting responsible for that
[HKEY_USERS\S-1-5-bunch of numbers here-1290584738-1000\Control Panel\Personalization\Desktop Slideshow]
(which are 10 sec and 5 sec in hex)
But it didn't work. Moreover, after reboot Windows corrected setting back to 10 sec.
I guess you wold have to use third party software for changing desktop images faster than 10 sec.