This is not a case of copyright infringement. Every photograph used in electronic medium is protected by copyright from the time it is created with the help of electronic fingerprints. This technology helps in searching the image, even parts of it, and then check if the image is used as licensed or not. Of course, as explained, royalty free images indicate that the license fee has been paid once to the agency or photographer that holds the license.
And, once that is done, the image can be used as many times as required, in keeping with agreement of usage. How much will it cost? Well, that depends on a number of factors like the size of the photograph, the number of times it will be used, the purpose of usage and other factors. So, using free stock photos is a cost effective way to use good quality images for your blogs, websites, presentations and many other personal and business purposes. Also, this is a legal way where you don't need to worry about infringement of copyright.
There are basically two types of stock photos - royalty free images or free stock photos and royalty managed images. We have already discussed what the first category includes. The only difference between royalty free and royalty managed images is that the latter's usage is limited according to the terms of the license agreement. The limitations are imposed on usage, industry, region and other similar factors.
With the advent of internet and free sources of images, we were introduced to the concept of public domain in a larger dimension. When we are talking about free stock photos we are still stressing on the requirement of purchasing the license of usage from the agency that keeps a stock of such photos. But in case of an image in public domain, there is no such restriction. Such an image is not owned by anybody and there's no question of buying the usage rights. Again, just because you have found an image by searching the net it doesn't mean that the image is in the public domain. You need to do proper research and see that if it is protected by copyright or if the license needs to be purchased for its usage to save yourself from any unnecessary legal hassles. It is to avoid all these issues that royalty free images are best bought from websites that keep a collection of free stock photos, with proper license of usage for future use.