Whene images are included in a template or signature which are inserted by
SmartTemplate4 / Stationery the images do not show, instead there is a
notfication at the bottom of the screen to whitelist the image.

"THunderbird has blocked a file..."

this relates to [Bug 1151366] a Thunderbird bug which addresses security
concerns regarding remote images + documents:

https://bugzilla.mozilla.org/show_bug.cgi?id=1151366

My theory is that we need to internally flag any included content like inline
images as "safe".. Additional challenge is to do this without re-opening
security holes.

Created an attachment (id=8296) [details]
1.5 prerelease 35

fix DataURLs from other template (Stationery)
This won't work if there is no "file:\\\" portion given (relative path /
current folder not supported)
we can fix the Data urls for file:/// images now

assumption: the URL is enclosed in single quotes, double quotes or terminated
by >

example: 
<img 
src="file:///C:/Users/Axel/Documents/Thunderbird/Signatures/thunderbird_blog2.png"
>

not working when we use:
<img  src="my-icon.png">
(which refers to an image in the same folder as the container file, e.g.
signature file or Stationery HTML file.

Setting to Resolved Fixed with official Release 1.5