The below links take you to resources that you can search for images. These resources are availble freely on the web, so you can access them from a basic web search, or click the links below.
Europeana over 6 million digital items from libraries, museums, and archives across Europe.
Internet Archive Book Images Images sourced from books that are now freely available for use. Date range is from 1500-1922.
World Digital Library: cultural images from around the world, sponsored by the United Nations Educational, Scientific and Cultural Organization.
World Images (California State University) contains approximately 80,000 images, is global in coverage and includes all areas of visual imagery.
More basic, and somewhat less reputable sources for image searching.
Compfight - this tool will search for images based on your search terms and label the resulting images based on whether they are freely usable under Creative Commons licensing or available for purchase.
TinEye Labs - Multicolr Search Engine - Search for images based on up to 5 colors. All of the images in this collection are usable under Creative Commons licensing.
OAIster: Find the Pearls (OCLC) search digital academic collections and select 'Image' under Resource Type to limit your results to images only.
Wikipedia Commons publicly available wiki with media files