Since we're just starting here, we need to set the tone when it comes to using external images. I've seen two answers where an external image was embedded without any link to the source. (I've edited the answers to include a link to the source, but the edits need to be approved.)
It may not be legal at all to embed such images (if they are not in the public domain) or even CC-BY images without proper attribution. So, please (1) use images that are legal to use (2) at least link to the source.
(See also this related discussion on meta.spanish.)