A few days go by, and the account takes down the post with your stolen artwork on it. You don't hear from them, so you assume they saw your message. The problem seems to be resolved for the time being, so you continue focusing on your own account.
After confiding in your friend about what's been happening, they suggest adding watermarks to your posts so no one can take your artwork again, but you aren't sure if you want to add them. On one hand, they'll stop the reposting problem. On the other hand, however, they can take away from the overall artwork.