"Print screen" (screen capture) can be useful when what's on your screen is not generally accessible or there's no direct link to it, but neither of those situations applies to the particular example of a 1900 U.S. Census sheet on FamilySearch -- as you said, you can just provide the link (yes, FS requires a login, but it's free), and/or you can download and save the entire image, using the "download" button above and to the right of the image.