For Markdown, I think you should check out Markdown Pad. I just saw it while researching about this, downloaded it, and it is really easy to use. You write as you normally would and then just highlight whatever text you want and click - bold, italic, or what-have-you - and the editor automatically wraps what you highlight with the correct Markdown syntax, and you can see a live preview in the right pane.

It handles images and everything else like that. Super easy, and then you can copy/paste that into the Steemit post editor. With images, it asks for the image url, so that gets included when you copy/paste . The only thing I noticed that it does not handle are videos.