Depends on who posted it and what they posted.
If I post pictures that I took, no.
If you post pictures that I took without my permission, yes.
If you post pictures that I took with my permission, no.
If my permission included terms, such as a copyright notice staying on the image and you circumvent those terms, yes.
If the media is public domain, no.
If the author states blanketly that anyone can repost anywhere, no.
Of course, in practice it's really a matter of--once you post something to the Internet, it becomes viewable by everyone the world over who has access. Enforcing copyright as the work proliferates across the many media that the 'net constitutes becomes VERY hard.