When Wikipedia calls itself the "free encyclopedia", that doesn't just mean there's no charge for admission. This also means it should be free of legal restrictions, and that makes image use policy very tricky.
On Wikipedia, there are three basic legal classes of images; knowing these will help you understand if an image you might want to add is acceptable:
Public domain: If the work of the U.S. federal government, or old enough the copyright has expired, it's OK to use.
If you've ever wondered why Wikipedia sometimes has terrible celebrity photos, now you know!