Some photographers and artists offer royalty-free stock images, including ones of horses. The copyright license determines whether or not a stock image is available to use at no cost.
It is best to contact the image copyright holder before using any image online, due to the number of different licensing structures. Some Creative Commons license include the CC BY, which allows for the highest freedom of use, and the CC BY-NC, which allows the same usage as CC BY, with the exception of commercial use. Sometimes an image provider may claim an image is using the Creative Commons attribute, but the image likely still has restrictions on its use.