Terms of Use for Images at WPClipart
Terms of use
These images are public domain (PD), and that means they can be used and edited for whatever purpose you wish, personal or commercial. No attribution or linking is required.
(although a link would be nice...)

You can even apply for Trademark status in the US starting with a PD image.
I only make one request:
I don't want folks to copy the collection to repost online (basically mirroring my site.) I spend a lot of time finding, researching copyright status and editing/retouching as well as donating hundreds of my own images, so I believe that should warrant some consideration. We can be courteous to each other, even online -- why not?
Allowed Usage Summary: