Although material may be copyrighted, museums and other cultural institutions are making image collections freely available online. Be aware that not all Open Content images are licensed the same way. Always double-check the usage rights for any image that you plan on using, no matter the nature of your use.
Use freely-available internet images with caution. Images may be too small or scanned at too low a resolution to be useful. Many web images are in volation of copyright or have been altered in their proportions, color or content.