If the image isn't signed and doesn't come with a text file claiming ownership, you're left guessing. In this case, you're at the mercy of the site where you found the image. It is wise to check the site for a statement of integrity, an assertion that the material was created by the site's owner, is in the public domain, or is otherwise available for use. If you're unsure, e-mail the webmaster with your concerns. If you're still unsure, play it safe and don't use that material.
Free fonts can be tricky, too.
Although a font may look perfect at first glance, you may find that it is not that great once it's installed on your computer. You may find, for instance, display fonts that only feature uppercase letters. Or fonts that lack less frequently used characters like the caret and accented letters.