Reading archive rights statements
The Library of Congress marks many photographs 'No known restrictions on publication': you can use them freely, and crediting the collection is a courtesy. Others say 'Rights status not evaluated' or 'Publication may be restricted': those are shown as 'check the terms', because a photographer's estate or a publisher may still hold rights.
Maps and posters
Maps published before about 1930 are public domain in the US and make striking backgrounds and illustrations. Posters and advertisements of the same age often are too, but trademarks in them (a brand name or logo) can still matter if you use them commercially.

