The current approach supported is to use image data tags, you could either pre-encode your image and include the data or use a feature of JEMH (Static Resources) to do it for you
<img src=”data:<MIMETYPE>;base64,<BASE64_ENCODED_IMAGE>”>
Here is an example of how we can 'inline' an image in the above way for a resource with ID 27.
$jemhUtils.renderStaticResourceImage($issue.getProjectObject(), 27)
Alternate paths to consider are externally hosted image files, eg from a CDN:
<IMG SRC="http://cdn..../image.png">
The following site adds some more background to approaches and highlights how no one solution is perfect.
As yet JEMH doesn't have the ability to attach 'referred' Static Resources' via CID image embedding, that improvement is being tracked internally as JEMH-7164OPEN .