But one tiny problem is still there:
I would like to include an image to the help page which is written in markdown. I tested it on my Mac, it works. Then I reinstalled the bundle and the image disappeared?? I used the following syntax:

Should I use the complete path to the image? And if yes, how can I get it? Or is it an issue of access rights?
The problem is that the base URL for a command will be something like tm-internal://… so when you reference a file with a relative path, it will be relative to that.
So yes, you would need to specify the full path or use something like this, just before calling Markdown.pl:
echo "<base href='tm-file://${TM_BUNDLE_SUPPORT// /%20}/
help.markdown'>"
That should make all paths relative to your help document.
As always, thanks you very much for the hint. I knew it that I've to set the base url, but I didn't know the syntax of 'tm-file'
Best,
Hans