Well, according to copyright law and social media policies you are only allowed to post your own work, works from the public domain, works that qualify for fair use (a pretty vague definition) or other's work with permission, including works with a creative commons license. This doesn't apply to retweets, Facebook shares, reblogs and all means of distribution of the original work, where the source is the owner's social site, of course. Some artists publicly state that they encourage posts and shares of their work, if it is properly credited. It means you can post their work without asking for permission every time you do so, in exchange for a correct attribution. That's exactly what we do: we love seeing our work shared and admired online, we just need you to show some support by giving us credit to help other people find us.