There's no way to prevent theft that doesn't also damage your ability to get feedback on your art. I'd say the benefits outweigh the risks with pixel art, because there aren't many commercially damaging ways for it to get stolen.
Art theft in video games doesn't tend to end well for the thieves, because people on the Internet are scarily good at spotting theft, even when the artist whose work was stolen isn't very well-known.
If posting your art on a public forum makes you or your company uncomfortable, you could try more private spaces, like Pixelation's Slack server, or over private chat with friends you make on forums like this. You could also delete the art from your thread after you've gotten the feedback you need - sure, it's probably still available somewhere, but the kinds of people who are too lazy to do their own art are usually too lazy to go digging for deleted images, too.
This is tangential, but how is it that your job has a marketing guy, but doesn't have you under NDA? Companies usually master the art of the NDA long before they can afford marketing people.
