Photographs and other images are more often the subject of copyright infringement due to the ease of republishing on the Internet.
Because images are so often published without attribution, it can be difficult to locate the actual owner of an image's copyright. Difficulty in locating the owner does not, however, excuse copyright infringement.
There are a number of stock photo organizations which license photos. These organizations have numerous licensing plans, ranging from one-time fees for unlimited use to royalty-based systems.
Most databases and websites provide information about copyrights and how their images can be used. Read this information carefully and comply with usage guidelines. Usage guidelines can vary considerably, so read the fine print.