There are a number of sites who host repositories of open source photographs that often come with a creative commons license. Most often, the photo artist simply wants credit for the photo in the form of a link back to their site or other way of contacting or crediting the artist. Different photos have different requirements for what you are allowed to do with each image, so be careful to read the creative commons license so that you understand how you are supposed to correctly use the image as well as what you aren't allowed to do.
Photopin.com is an excellent source of free images because it has a pretty good selection of photos and it makes it extremely easy to follow the creative commons rules. Photopin provides the HTML for you so that all you have to do is copy and paste the artist credit at the bottom of your post or article and you have complied with the license for that photo.