U.S. copyright laws protect the creators of original works, including photographers, from unauthorized use by others. No matter what medium you're working in, you must have permission to reproduce a photo if someone else created it. Simply acknowledging the artist does not substitute for permission. If you don't get permission, you could be liable for copyright infringement and assessed a fine enforced by federal law. There are legal ways to use another's work without violating copyright laws.
To obtain a license to use a photo, you can sign a contract with the photographer. This can be for one-time use or multiple uses; the photographer can limit the medium in which the reproduction appears and the geographical region of distribution. Professional photographers usually use standard contracts and rates. They can supply you with a form, which you can alter only with their agreement. Although most photo contracts use a flat-fee structure, you can also sign a royalty contract if you're working in such media as posters, T-shirts and souvenirs.
If the photographer licenses the work through the Creative Commons system, then you can use the photo for free. Creative Commons has several license types -- selected by the photographer -- which control frequency and method of use by others. An Attribution license, for example, allows you to alter and distribute the photo as you please, as long as you attribute the photo correctly. The Attribution Noncommercial license limits the use to not-for-profit media, with attribution. Finally, Attribution Non-Commercial No-Derivs means you can use and share the photo, but must not alter it in any way or use it for a commercial purpose.
You can hire the photographer as an employee, who then works at your direction and follows guidelines you set as a publisher. In an employment relationship, you can set the compensation terms, but also must follow IRS rules for reporting of wages, as well as income and payroll tax withholding. In this situation, you, as the employer, own the rights to the pictures your employee takes under your direction. Employees are not the same as independent contractors.
Work for Hire
If you engage a photographer as an independent contractor, this work-for-hire relationship means you've hired a freelancer for a temporary assignment; there is no employment relationship. Compensation is set by a contract. You can set the terms of the work and also control the use of any photographs that are published. In effect, you control the photograph copyright, although it may continue to legally reside with the photographer. Many photographers agree to work-for-hire contracts only if the term of use is limited, for example, until a book in which their photographs appear goes out of print.
Your reproduction may also fall under the "fair use" doctrine, which is a defense to a charge of copyright infringement. According to federal copyright law, fair use includes the use of a photo for educational purposes, research, criticism, comment and news reporting. Fair use may also apply to the incidental appearance of a photo, such as in the background of a larger image. One factor in determining fair use is whether it is of a commercial nature or is for nonprofit purposes. Whether the fair use doctrine applies to a particular work depends on several factors, and each case is looked at individually.
A photograph may also have entered the "public domain," where it is not protected by copyright law. Photographs taken after 1988 enter the public domain 70 years after the death of the creator; prior to 1988, copyrights expired 50 years after the death of the creator. All photographs belonging to the federal government are public domain as well, although each agency -- notably the Library of Congress and its huge photo archive -- can charge a use and reproduction fee.