So my plea is a simple one, but heartfelt. Please, if you're using someone else's photography/textures/brushes or any other resources to make your art, read and follow their rules. It's SO easy, and so worthwhile.
Sure this is the internet, but you have to remember that it simply would not exist without its users; you, me, and them. They're all people! I know, it's easy to forget that. And the vast majority of them are sharing their images for free, their only reward for which is seeing how their work has been used.
Now I don't want to get too preachy about this, but stop and think for a moment; if you shared something of yours with a stranger or a friend, for free, and then found out they'd taken it, claimed it as their own, and maybe even sold it... how would you feel?
In summary: Credit your resources. A tiny effort for an enormous reward.