In an answer to a recent ELU question, two images of ngrams were included without live links to the ngrams site. I believe that links should be included because it's desirable to be able to try variations of ngrams searches without having to start from scratch typing in all the words. (Note, the answer was updated with links during its first hour.)
Is it reasonable, or not, to downvote a question because of missing source links? (Or, more particularly, missing ngrams links?)
Note, I'm not suggesting leaving out images. With images included, ELU answers remain intelligible even if linked sites disappear, and that is good reason to include them, distinct from another important reason: rapid and accurate transfer of information.