I was contacted by someone earlier letting me know that one of my pieces has been posted on tumblr without the proper credit. I don't mind my work here being posted on tumblr as long as you use the share button that deviantart has which automatically puts the credit and a link back to the artist's DA page underneath. This has already been done with some of my pieces and I'm fine with it.
What is not okay is just taking the image (by saving to your computer and posting or just copying it over) and posting it with no reference to the original creator. That is effectively theft. I make no money from the things I paint and I post them on the internet for people to (hopefully) enjoy with no cost to them and no monetary benefit to myself. I benefit instead via the recognition and encouragement from those who see it which enables me to acknowledge that the time and effort I put into something was worthwhile, thus encouraging me to continue. Reposters undermine that effort by claiming something as their own. Even if they do not explicitly claim that they are the original creator, they are lying by omission.
So please, if you ever suspect an image to be a repost, contact the reposter and kindly ask them to remove it or post it in a manner that sufficiently acknowledges the original creator.