A question about posting reference photos

Hi,
I've noticed that sometimes when a person posts a sketch or a drawing, he or she will also post the reference photo they used.It got me wondering, what if the reference photo you used came from a book, magazine, or brochure? Would the poster still be allowed to scan and post the image? Or is it better not to because of any potential legal issues? Just wondering.

As long as you arent saying its yours you should be fine. Just give credit where credit is due.