Yeah, if I physically stole the artwork, then yes? That's stealing. Let's cover all the bases, though, shall we?
If I took a picture of some art, is that stealing? No. That's not illegal anywhere.
If I then shared that picture with a friend? No. That doesn't count as a "public performance" or "public display" of the work.
If I then shared that picture with the Internet at large? Yes, that's copyright infringement—every time someone downloaded it, that counts as one violation. Specifically, derivative copyright infringement (which is a whole world of law in itself).
Aside: The RIAA argued in court that merely placing one of their copyrighted movies/songs on the Internet counted as distributing their work to the entire world's (Internet) population and therefore, the defendant should be held liable for billions of copyright violations for each work they merely made available (who cares about the actual number of downloads, right? LOL). THEY ASKED FOR $72 TRILLION IN DAMAGES:
Saying "piracy is theft" is playing right into their hands. Unless you're a paid corporate shill/bot, you shouldn't be making arguments on their behalf. At this point you should be asking yourself, "Are we the baddies?"