strictly speaking, even royalty free images would not suit your purpose... someone still owns the rights to royalty free images and if the rights holder disagrees with their usage you could find yourself in trouble. That said, its a bit like taping the simpsons from television. It may not be strictly legal but the feds are not gunna come round your house for it. If you really want to be beyond reproach - make the images yourself, use non pirated software to publish them and do not accept any money (yes, sadly, that includes donations). For this type of deal rights holders shouldnt really have any problem with it it as long as you ask in advance, breif them on your plan and credit all work not done by yourself.
But remember, ^that is being super anal/legal and this is just a hobby.
good luck! any finished skin for xbmc is a good skin.