Stock photographs are licensed for a specific use, such as creative or commercial work. Licenses, which can be purchased from a vendor, allow you to legally use stock photos. There are many online vendors from which you can purchase stock photos, such as:
Warning about Usage: Search Engines and Image Libraries can help you find images, but that does not make them yours to crop, edit and Photoshop. Many images are copyrighted and for personal use only. Some are shared with limited creative commons licenses and some rights reserved. Some search engines, such as Google, allow you to filter results by usage rights.
Always read copyright policies to determine whether a Website allows downloading/ use of its images.