Before buying stock photography, make sure you read the license. In early stock photography contracts, people would pay for "print runs." You would buy a right to print a given number of copies at a given time. This does not work for the Internet.
Royalty free stock agreements are more generous in letting you include images on electronically generated pages. This means you can put the image on your site without counting image views. The contracts, however, balk at anything that might be construed as reselling the image. You could include the image on a corporate brochure, but could not sell greeting cards or sell t-shirts bearing the image.
Netiquette requires that you credit artists for their work; So, I made a Microstock Crediting program to help manage projects (more)