Don't think it's possible. I think I tried to do that pasting in my HTML couple of times but steemit seem to remove them. I think there's also a strange bug where if you are writing a post, you switch between the Editor, Raw HTML or Markdown options, it messes up the flow of the text and images sometimes. There were times I preview, looks good right, post. Then check first part OK and following the images missing but were there all the way at the back.