Add profile picture for guest byline instead of using Gravatar

Just an idea.

At the moment, it seems like the only way to associate a profile picture to a newly created guest byline is to add an email address that is connected to a Gravatar account—just tried adding my email address connected to Gravatar, and pulled my image from there successfully.

I think it's good that we can use Gravatar, but it'd be much better if we could have also the ability to upload a profile picture, just in case a guest contributor isn't on Gravatar.

@roborourke thoughts?