Clip art is available from software companies and websites. Before using any clip art, you should read the “Terms and Conditions” carefully, as the licences limit the ways in which the clip art may be used. If there is a statement that says the uses of the clip art are limited to private, non-commercial uses, then you should seek permission before incorporating clip art into anything you are creating for commercial purposes.
You should also note that a person cannot give you a valid licence to use clip art unless they own the rights, or have obtained the permission of the copyright owner. If you consider the copyright owner may not have granted permission for the image to be made available on the website, you should not use it.