Thread:Simon Peter Hughes/@comment-24520859-20150523105254/@comment-4142476-20150525230344

Well as far as I know, it has to do with three elements:
 * smaller portion (you're not using the entire thing, only sections)
 * lower quality (low-res version for hi-res images as you said)
 * purpose: educational, etc.

But I'm not a copyright expert.