When you're including an image that's served from the same domain as your page, use a site-root-relative URL (starting with "/"). Use this URL format (starting from the site root) even if the image is in the same directory as the page that includes it.
HTML
Insert the URL in the src attribute of your img element:
<img src="/shared/images/arrow-24.png" alt="Alt text description of arrow image." />
Markdown
Insert the URL in parentheses after the image's alt text:
