art work present on mac... but on PC - only some images appear

[ferd]

I have a library of tunes to which I've manually added images - on the
mac they're all there and it looks good.

when I take the usb drive over to the PC and select that library as the
one to use in iTunes (both 7.02 on the mac and pc)... some of the
images appear, and some do not. most of the images were added by
dragging an image into the artwork tab on the "get info" area of a
tune. some were a copy-and-paste job, don't know if that has anything
to do with it.

question
- should I have done something on the original artwork insert to insure
compatibility?
- is there an easy way to assign the images (which i presume are there)
in the artwork folder so the PC sees them?

any insights appreciated!

tia
ferd