Images can be hosted from the /content folder in your documentation project.
To write just the url of an image when you want full control of the html, use <[url:content/images/spec-explorer.png]>.
To write the image tag with a header capture, use the syntax:
<[img:content/images/editing/status-bar-success.png;Status Bar with a Success]>
The syntax inside the brackets is [relative path to the image file];[caption title].
See the topic files for the Storyteller user interface here and here for example usages.