Images, like written words, have copyright unless the creator of the image releases some of their rights. To release copyright people can post images using a Creative Commons License. If they use a Creative Commons license they can decide whether or not others can use their work to make money and/or make changes or remix their original images. When using Creative Commons licensed images (or other works) you should always provide attribution to the original creator of the work.
To learn more about the Creative Commons Licenses click here.
To find images with Creative Commons licenses you can use the following resources: