Um, I guess @Alex9Wolf9 mean to put picture directly in the novel, not banner. You could actually put picture on your novel if you have firefox by copying the image and paste it to your novel (not ctrl v). I don't know any other methods, and never tried it since I don't have pc, but peoples said its work pretty well if the size is not too big.
As for if its allowed or not, I don't know, ask @STAFF, but it may be allowed, if its going to be like a light novel or some sort, but eh... idk