I am also having this issue and after much forum searching, Im still struggling:
I have the Raspian image and using the sudo raspi-config command i have it set to boot straight to the GUI Desktop.
I have then edited the following to start Midori with full screen:
Sudo nano ~/etc/xdg/lxsession/LXDE/autostart
@midori -e Fullscreen -a http://www.example.com
I have done what you suggested and have added the following to
sudo nano /etc/lightdm/lightdm.conf
Under [Seat Defaults]
xserver-command=X -s 0 dpms
However, my Pi still blanks after 15 mins, do I need to add this to that autostart file?
If so, any ideas how?!
Would appreicate any help you can give!