Hi, Benjamin! Thanks!

There was also DC update last week Paul mentioned about new button in WYSIWYG editor. 

You can click upload button (red circle in a screenshot) and image will be uploaded to your message in the place of cursor.

Of course it is good to stretch it to 600px - you can do it if you select all the image and click image button (red rectangle in screenshot) and set image's widh.

I did it to the screenshot of the post.