The federal government, by nature of its vast scope, is a huge producer of maps, geospatial, and visual content. It has done so since the beginning ot the nation's history on both Earth and in space. Any visual resource made by the federal government is in the public domain (not under copyright) and free for you to use in assignments and projects, although attribution is still best practice.
Picture of Neptune as taken by Voyager 2 spacecraft (1989)
PolicyMap is a U.S. national data and mapping tool that combines a curated, geographic data library with simple, robust mapping and analytics tools.