If you're looking for backgrounds featuring actual NeoPets and items, you're not likely to find one (or at least one that'll be around for terribly long) because redistributing NeoPets images (derived, altered, or otherwise) is highly illegal and the NeoPets legal department can and DOES go after sites for doing that.

If you're looking for just any ol' Halloween background, there are tons of sites out there with copy-and-paste codes for them.