In order to protect a person's privacy, you are not allowed to use images or videos of people for commercial purposes without that person's permission.
In order to have such a permission, there are two options: 1) There would either need to be such a permission already in place, or 2) you would need to enter into an agreement with that person called a model release. That means that the person in the image or video would grant you the right to use that product commercially.
The same principle applies to properties, where the owner of a property (for instance a house, a car, a factory, a business, an object of art, trademark, design, etc.) will have to grant permission in order for you to use a media product containing that property in a commercial manner.
However, most media products that are available through the stock image websites reviewed on Topimagesites, already have model and property releases in place. This means that when you find an image you like which includes one or more people or properties, this image will come with an already signed release. When you look at the image preview on most stock image websites, it will state whether these licenses are alreay in place. It is only in rare instances that these licenses are not obtained.
The media products that do not have a model or property release can still be used without getting a model or property release, but only for what is called editorial use. To learn more about editorial use, please go here.