The Public Domain Mark is recommended for works that are free of known copyright around the world.
[1] For a creator to release their works into the public domain legally they must use the creative commons CC0 license which gives creators a way to waive all their copyright and related rights in their works to the fullest extent allowed by law.
[2] The presence of a resource on this list does not guarantee that all or any of the images in it are in the public domain.
You are still responsible for checking the copyright status of images before you submit them to Wikipedia.
Clip art is generally composed exclusively of illustrations (created by hand or by computer software), and does not include stock photography.