Creative Commons Image Search allows you to search several image repositories (including Flickr and Google Images) and limit your search to only those images which are licensed for others to reuse them.
VROMA's Image Archive collects images from museums all around the world, focusing on artifacts and depictions from ancient Rome.
Always check the copyright terms of any image you find. Sometimes the terms vary with each image (as with the ones you might find using a Creative Commons search), and sometimes the terms will be the same for every image on a site (as with the images on VROMA). You are looking for images whose terms allow others to reuse or redistribute it.
Make sure you can attribute your image properly (give the name of the artist, photographer, or creator) and find out as much as possible about where it came from.