The company which owns the copyrights sets up a business unit to collect fees from unsuspecting infringers. They have software which continuously scans the Internet for web pages which contain photographs in their database. If your web site has an offending photograph, the odds are very good that you will be caught.
The problem, from the user’s perspective, is that most users who are caught in this trap have no idea they are using copyrighted photos, and have no intention of infringing anyone’s rights. In fact, they may have downloaded the image from what they think is a free site, from a Web site which says the download is free, or from a web page which says nothing about copyright. The web page may say that there may be copyright rights, but gives you no hint about how to find out who owns the copyright rights.
The simple fact is that every photograph has some copyright rights attached to it. Once you are “discovered”, the copyright owner sends you a letter demanding that you cease the use, and naming a demanded amount of payment. The typical demand amount is at least $1000. As in all cases, be sure to seek the advice of a copyright infringement lawyer prior to moving forward with any sort of action.