A list of sources for public domain and relatively unrestricted images.
To find books rich with images, add the words "pictorial works" (in quotation marks) to your keyword search in the library catalog:
horses "pictorial works"
Searching by image to find that image online:
TinEye (http://labs.tineye.com) - Click on Search. Upload, paste, or provide URL to do a reverse image search.
Google Images (http://images.google.com/imghp?hl=en&tab=wi) - Click on the camera icon to do a reverse image search. Paste URL or upload image.