Nothing comes without risks, especially in an age where everything is share on the internet. It's all a gamble, but you can reduce fear and worries, by choosing what and where you share.
For me personally, I never had any major problems: I've been taking art and writing commissions for the past 7 years now, and not once did I faced art theft. But I've limited my sharing to Twitter, Wattpad and ScribbleHub only. And surprisingly, I've had more than a handful of people requesting for me not to share their commissioned work, which of course, I obliged to every time.
To be honest, I'm a tab more worried about my stories being stolen, than my artwork, but again, it's a risk I'm willing to take, in order to share with the world, my countless and sometimes overwhelming ideas.
Hope that can help a bit,
Kindly, Rin.