I think it's the artist's job to let it be known that they are not friendly to reposting their work. If you want to be that weird 1% of artists who posts their work publically on the freaking INTERNET and you don't want others to share it around, then that's your job to voice that considering that's not really normal.

That said when you do repost their art you had better credit them properly and source back to their site otherwise you are a garbage person.