This site is self-described as such:

The intent of this collection is to support the teaching of American political history by providing quick access to uncopyrighted images for inclusion in teaching materials.

All images are strongly believed to be in the public domain. They were obtained from non-copyrighted U.S. government holdings and publications and from published works with clearly expired copyrights. Thus there are absolutely no restrictions on their use.

Images are provided in two sizes and in JPEG format for easy retrieval and use. The small size averages 400 pixels in the long dimension and 100k while the "full size" averages 1200 pixels in the long dimension and 320k. Note that the file sizes are rather large for JPEGs. Low compression was used so that these images could be edited and recompressed by the end user. As a result, download times will be slow with 56k modem access.

Contains over 500 images.