I just uploaded to IMGUR, then right click > view image. Save that URL.
Come over to steemit and submit a story, make sure you click "editor" on the right side of the text area.
There will be a tool box that opens up, and there is a IMG logo (last one on the right next to text size)
Enter the URL there, and press the checkbox.
To preview it, just press "Raw HTML" where "Editor" was, and it will show you the HTML code.
The preview will show up at the bottom of the page.
TL;DR - Use HTML code
Hope that helps.