Publishers or studios that buy your art or time own the work you create and can sell, distribute, or license it as they see fit. On Storybird, the art you upload isn't being sold to us. You're just licensing it to us so that we can help you promote and sell it to our members. In that respect, there are no conflicts: you own the art and can sell it anywhere else you please. (You can also remove your art from Storyird whenever you want, whether you're just pruning your shop or shutting it down altogether—although pre-existing stories with your artwork will still remain on the site.)
Commissioned art—unlike the "stock" art you upload—does have some conditions: it's exclusive to Storybird, remains on our site, and can't be displayed or sold in other channels.