Whether the comic is online or printed, there's no difference. Work is work. When you write a script, there's no difference to the craft of writing for the Web or print, right? Then, it's the same for artists.
@LeaLeeLee mentioned that many artists have stories to tell. Just make it clear, that it's your story, not theirs. Also, do discuss the issue of copyrights with your artists.If you pay them, there is an assumption that it's work for hire and that they have waived their copyrights and any ownership to the work.
Not pleasant things to discuss when you're all hyper about a new project, but something necessary to address for health and viability of your project.