You can't use anything trademarked or copyrighted by anyone else. Sometimes it's not easy, especially on the internet, to confirm exactly who owns the image and check with that person for copyright/trademark, but a good rule of thumb is: you can't use it unless it specifically states that it is in the public domain or available for use. Any photo a person takes, art someone creates, sketch, drawing, image, etc. will by copyright that person or his/her employer (or possibly of a third party, if they then sold the image), unless the owner(s) specifically made it available to public use. And clip art in particular is almost *never* available for free use: people make money on clip art by licensing it for a fee for certain very specific uses or purposes. If you want to have images you can safely use for certain, have your own images made (e.g. take your own photos or hire a photographer; create your own drawings or hire an artist; etc.).