Copyright

Before using any image you find on the web, make sure that the owner does not object.
Site authors often provide a statement concerning how their work can be used. Follow their guidelines.
Even if an item does not have the copyright symbol on it, copyright does apply.
Unless the site specifically states "free clip art" or something similar, contact the site author to determine if/how you can use their work.
On most image search engine sites, users are linked to the originating web site on which the pictures are located.
Should you wish to use any picture, photo or artwork you see during the search process, you must obtain the appropriate permission from the owner of the material.

At the very least, be sure to correctly cite where you got the image.