I think my question says it all (and yes, I've googled and searched StackExchange already) - but to clarify:
I want to embed an image, which I have locally, in my html email signature (i.e. my logo, linked to my website). I create a html file (and made a web archive of it) with referencing to my local image like
Opening the web archive/html page in Safari displays the logo fine.
Now, when I use this html signature, Thunderbird is smart enough to embed the local image properly with a binary mime type of image/jpeg, however, Mail just doesn't do anything with it and just leaves the html code untouched, still referencing to the local file (and even that doesn't work/display when I receive my own mail on my Mac where I actually have the file at that local place; probably sandboxed?!).
I've already googled and all of below are not working:
(and their respective linked articles on the web).
The only way I could get the local image to display in the received email is to drag & drop the image from the Finder into the Mail signature preferences window, but then, I still have the problem that the image is not linked to my website, and I see now way to edit the html source of it in any way.
I've already tried to drag & drop the image, then open the web archive file under /Users/myuser/Library/Mail/Signatures in Safari, fetch the html code (which holds code for an embedded object), modify the source code (adding a link around the image), save it as web archive and overwrite the old file. However, after doing that and restarting Mail, the image doesn't show, but instead shows a place holder saying "Missing Plug-in".
(I don't want to use remote images on the web, since many clients mostly ask users to display remote images firstly, before actually displaying them - embedded images therefore work better, as they will be displayed instantly in most cases/email clients' default configuration.)