1, the hack website? all the different catagories of backgrounds on it? i know of that one.
2, the only space custom backgrounds take on the mem card is when you put them on there for pspersonalize to use. once it stores them on the PSP, it copies over the backgrounds in the psp, they are no longer needed on the mem card at all after that you can delete them.
problems with them are because of thier size, they dont work perfectly, if you switch months, you might get an all white screen, which you then have to goto the change month and click on it again to make the picture come in. It also goofs up some icons like the currupt icon (which isnt important if you got rid of the currupt ones on yours)
3, yes i make them in photoshop with the right size and save them as a png, and pbp maker lets me switch the icon and background in the boot.pbp file, so it works well, i suppose i could also put in a sound like the PSPersonalize one has if i wanted.
4, what ghosting problem? protect you from what?