Try doing a google search for "royalty free clip art" and then search again for halloween at the sites that come up.
I use to belong to a subscription service that offered quality clip art. At one point you could order it online but I can't seem to find them any longer. When desktop publishing was first becoming popular there were all kinds of clip art sources available on CD also. I know Dover Books use to publish printed books with clip art usually organized by subject that you could scan and use. Sorry this isn't more helpful but maybe someone will see this and it will trigger something for them.
What kind of images are you looking for anyway, non-kidlike covers a lot.