Right after you have typed pixabay.com into you browser you will see this

Then type in a niche or subject in you will see something like this - (In this example I will uses weight loss foods but you may use anything that is suited to your niche!)

Notice the photos at the top are on a yellow background and are watermarked yes? Well these photos have to be paid for and the rest do not ,so, ALWAYS stay away from those if you want your browse and the photos you pick to remain free.