Royalty-free stock photography and illustration has evolved a long way since its inception as an alternative to traditional rights-managed imagery. It has moved from analog to digital, then to the web and is now provided in models ranging from single-image purchase to subscriptions. Typical conditions of royalty-free images include: 

• A one-time fee for the right to use the image multiple times for a a wide range of purposes, with such typical exceptions as use in software, templates, videos and computer games, or use as part of a trademark, service mark or logo. Use for defamatory, libelous, immoral, obscene, fraudulent or illegal purposes is in most cases not allowed. Use with mobile or wireless devices may also be prohibited.)
• Images usually can't be applied to other items for resale, license or other distribution for profit, such as t-shirts.
• In the case of some limitations, such as the number of times an image can be reproduced in a publication, an additional license is usually available.
• Copyright attributions are sometimes required. 

Tip: Never purchase royalty-free stock content without first carefully reading the license agreement.