it isn't only copyright that's an issue (although it can be).
It is also the Terms of Service of various websites. If you download an image from (say) Findmypast, you probably do not have the right to publish that image elsewhere, even if it is out of copyright (because by signing up to a website, you accepted Terms of Service that forbade you from sharing reproducing any downloaded images.)
Some people will shrug and assume they'll never be found out.
I choose not to publish any images of sources -- which is one reason why I take great care with my citations, so that others can locate images that I don't have the right to reproduce.