Regardless of where you locate an image, please remember:
Always give credit to the source from where you found an image as you would a text source!
Open Access, by AJC ajcann.wordpress.com, CC BY-NC-SA
Many cultural institutions allow downloading of public domain images of artworks from their respective collections. Public domain means no longer in copyright and is free to use in any way.
Images are all around us and are easily searchable thanks to sites like Google Images, Flickr, Tumblr, Instagram, Pinterest to mention a few.
Free to use images (copyright-free/public domain/Creative Commons licensing):
It depends upon your assignment whether these are acceptable ways of gathering and using images. Please consult with your professor if you have questions about whether or not these are suitable ways of searching for images for your given assignment.