Custom Thumbnails fore music folders??

Does anyone know how to set custom thumbnails for my music folders?

I have a folder for each album on my psp and would like to set a thumb for each folder.

I Right click on my folder and select properties, then I click the customise tab and select folder picture.

The thumbs work on my computer but not on my psp, Although some of my folders already have thumbs that where there when i ripped the CD's.

Maybe i have to have a certain size image?
I really don't know how to do it, but it must be possible because some of my folders have thumbs?

Any info would be very helpful,
Thanks,

Ghost DOG

if you usee itunes and set abum art to songs, then export to the psp you will have the pictures

Also if you use music match it will do the same.