Depending on where you live (and to be honest it covers most places), intellectual property is automatically protected the moment you create it in a tangible form - draw a picture, write musical notation, etc.
You don't 'need' to buy or register copyright, for that basic protection.
So if someone 'steals' it, they're already committing a copyright violation (depending on whether the use is fair or not). The only question is, what are you going to do about it?
A - you can file a DMCA takedown notice to have the image removed. Anyone can do this. People can even do it falsely.
B - if you want to file for damages, (not that it's likely to cause any real damages, but just saying) then sometimes you DO need to have that copyright registered. The US is one place that enforces this.