In general, you may not reuse images found on Google for clip art. However, some images are available for either commercial or non-commercial usage. To find such images, search on Google Images, select search tools below the search bar, then choose the appropriate usage rights from the drop-down menu.
There are many other ways to find images you may reuse as clip art. For example, try Flickr for photos. As with Google Images, you need to make sure you are only searching for images that you are allowed to reuse. To do this, click on Advanced Search. You can also try OpenClipArt.org, which offers high-quality clip art images for free for commercial use.