First of all, yes, I have delt with it myself. I have had art stolen from me before. And It pissed me off to see that my friends are getting their art stolen too. 

If you do not know the artist, do not use it. Even if your just going to use it for a profile picture. 

If the artist doesn't allow reposts, do not repost it. Respect the fact that the artist has put their own time, effort, and talent into it, and don't fucking post it. 

It is theft. It is illegal. & it is just plain out rude and wrong. Don't fucking do it.