Published clip art is every image you see online or in print. Unless you are dealing with a company instead of an individual or a blog, it could be difficult to find the source of these images. Unless there is a statement that invites you to use the image again, you must receive permission. While this is particularly important for commercial work, it still applies to educational, personal and non-profit use.
If you can't find the source of an image, don't use it. Remember, it is better to be safe than sorry!