In the US, everything you create is immediately copyrighted, even if you haven't published it. Your concept art is already copyrighted. You can pay a fee and send it to the copyright office to be registered and all that, so that they have an official record of it, and that basically means it's easier to contest in court if you want to take legal action. But I mean..... that amount of time and money might not be worth it for some kid tracing your design on DeviantArt.
Stealing original ideas really doesn't happen very much and usually doesn't matter even when it does. Gaining fans and fame and money through original work takes forever and a lot of hard work; people are much more likely to steal fanart because they know it has an audience. What would I get from stealing your ideas?
And what happens if someone steals your idea? There are sooooooo many stories with similar characters or similar plot elements to each other out there. Even if you do somehow run into the combination of someone who (a) sees your concept art (b) has artistic skill but (c) doesn't want to use their own ideas instead, and (d) has something to do with that design, and that unlikely person does base a character off of your idea..... so what? You can still make your character, and it's impossible for that other person to match the character you had in your head, so there will still be plenty of differences.
If you wanna post stuff online, you gotta let go of the fear of having ideas stolen, because there are two defences against stolen ideas -- the first defence is to never put anything online ever. The second defence is to put a lot of stuff online, because then you establish yourself, and establish your ideas as your own. Then, even if someone else does try to poorly copy your idea, it doesn't matter.
