Why is it that when I'm printing photos, they come out clear, but when I print drawings and oil paintings off Google Images, they come out slightly pixilated?

I am doing a craft project and saved a bunch of photos from a friend and also some images of oil paintings, drawings, watercolors, etc. from Google Images and other sites on the internet. In printing them today, the photos come out well and clear but the images come out pixilated. Why would that be?