Just to say for my model (PW high res) the screensavers have to be 1072x1448 and 120 ppp... they says when we reset it 300pp res... don't ask why ^^'
For others in my case who don't know what resolution is their model and have JB it and installed the screensaver hack, just go to the backup folder and check the res of your screensavers
Some probelms:
-the picture have to be black and white of course but also 8 bit colors
-I had to manually reboot and add a blanck file named radom and another one named shuffle
I hope it will help someone
see ya