Some content available online, such as public domain content, is free to use because it is not subject to copyright protection. Other content might be subject to copyright but the copyright holder licenses content with certain restrictions, such as under the Creative Commons license. Bing’s image search lets you limit results only to Creative Commons-licensed images (after running an image search, click “license”). Other copyrighted content may be used without permission because a limitation or exception to copyright applies (see above discussion of fair use).
Of course, some online content is not free to use, is not licensed by the copyright holder, and your use will not qualify as fair use. Unfortunately, we can’t provide specific guidance regarding the use of particular content, so be careful to select the works you use carefully.