I think you are good on this. Public domain means it is for public use- though with some characters like Capt. Marvel you want to be very careful in what you do. You are taking an image, and painting it. It's your interpretation of another's work. I would just tell you to be sure to credit the original artist along with yourself. There is a woman who paints entire pages from comic books (and they are not public domain ones, either) and she gives no credit to the original artists although her paintings are basically acryllic blow-ups of the originals. How she hasn't been sued by the publishers or the creators for everything she has, I don't know.