I am not a lawyer. You should get one, if you are concerned about possible legal issues.
First of all, pictures that are not copyrighted (i. e. in public domain) are not your only hope. There are quite a lot pictures that once you get them might be legally used for any purpose (incl. for profit), modified and/or reproduced without royalty, they are called ‘free’ (as in ‘freedom’) or ‘libre’ ones. The popular copyright licenses for free/libre works of art are: CC0, CC-BY, CC-BY-SA, Free Art License. (Do not forget to understand them, they have certain requirements.)
You might use, for example, Google Advanced Image Search (usage rights: ‘free to use, share or modify, even commercially’) to find free/libre images on some topic, but you have to be careful: copyright status might be mislabeled, you’d better double-check it with reverse image search and common sense. Some resources (like Wikimedia Commons) are quite trustworthy, some are not.
Also, a copyright of author of an image is not the only thing you have to be aware of: if it’s a photography of a person, that person also have certain right that varies with jurisdiction which you might violate; some words or (a name of a sport event for example) might be trademarks, et cetera, et cetera.