The heading elements <h_> by default is block-level, which means it spreads full line width. It can be assumed that Moon+ is merely holding to the standards expected for the <h2> element and making your image also spread full line width regardless of your CSS.
And...you likely don't want the image to be 1em tall - that's the height of the words on that line, and your image is a fully detailed designed image that can only be viewed when its larger than 1em.
So...set the image outside of the <h2> element, and use the <img> tag as Quoth suggested: width: 50%; height: auto; .
Read the following at Mozilla.
MDN heading
MDN block image