If nothing simpler works, you can try interweaving html directly with an @raw html block.
markdown code
```@raw html
<br style="line-height: 150%;"/>
```
markdown code
Afterthought #1: In your case, you might prefer to insert the <img> tag directly and set CSS margins to give a little more room?
Afterthought #2: If you want to apply similar spacing adjustments to all images, consider customising the documenter theme.