Unfortunately not working for me..
I'm on 5.3.1 Jailbroken PW with the custom fonts hack applied, I tried your method but it only displays the default one.. When I copy png's to the screensaver folder it auto renames them to bg_medium_ss00, bg_medium_ss01 etc.. but wont display any custom screensavers when I hit the power button, any workaround for this?