It's fine to quote excerpts from them, but make sure you follow our attribution guidelines. See the Help Center's page on how to reference material written by others.
Also, if your entire answer consists solely of a quote/excerpt/image from an external source, something is wrong. We don't want that kind of answer. Your answer should be your own. You can use external sources to supplement or support your answer, but we don't want to be just a link farm or an image farm.
It's better to transcribe the contents of the page to text, rather than using an image. First, that will make the page findable by search. Second, images aren't accessible to folks with visual impairments (and this is going to happen to many of us as we get older) and sometimes don't display as well on mobile platforms. So, transcribe to text.