Wanting to use an image from sxc.hu for an e-book cover...wondering what credit I need give?

I am interested in using a royalty free image from the sxc.hu site for an e-book cover. I am wondering however, what the rules and guidelines are for crediting the photo? I know nothing about this stuff, but thought maybe since it said "royalty free," credit is not necessary?? Crediting would not normally be a problem, but being that the photo will be on the e-book cover, I don't want to gunk up the look with unnecessary words, if I don't need to. Any help would be appreicated!!! THANKS!