Update: The relative url will be inserted when using 'Insert image' (upload) instead of 'Select image from server'.

Best is to make the pull request ? No ?

Here, you import external image too ?

Right ? Then : it can take a really more long time to send email …

Well, this is a solution that works for me. I personally don't want to have any external image in my mails. But it might rather be an option.
It shouldn't be slow when bug 15808 has been fixed in a proper way. The image will be added once, converted to base64 and sent to all addresses.

In order to prevent slowness, the images might be cached like this.

private $imageReplacements = [];
function msgHTML($message, $basedir = '', $advanced = false) {
    preg_match_all('/(?<!-)(src|background)=["\']http(.*)["\']/Ui', $message, $images);
    if (isset($images[2])) {
        foreach ($images[2] as $imgindex => $url) {
            $url = "http".$url;
            $key = "/".preg_quote($url, "/")."/";
            if (!isset($this->imageReplacements[$key])) {
                preg_match('/(?P<ext>gif|jpe?g|png)/i', $url, $match);
                if (isset($match["ext"])) {
                    switch ($match["ext"]) {
                        case "jpeg":
                        case "jpg":
                            $mime = "image/jpeg";
                            break;
                        default:
                            $mime = "image/".$match["ext"];
                            break;
                    }
                    if ($im = file_get_contents($url)) {
                        $this->imageReplacements[$key] = "data:{$mime};base64,".base64_encode($im);
                    }
                }
            }
        }
        if (count($this->imageReplacements)) {
            $message = preg_replace(array_keys($this->imageReplacements), $this->imageReplacements, $message);
        }
    }
    return parent::msgHTML($message, $basedir, $advanced);
}

My opinion are : Best in a core plugin :)

You can check https://github.com/LimeSurvey/LimeSurvey/tree/master/application/core/plugins/mailSenderToFrom for sample

1. Attach to beforeTokenEmail event only (in my opinion)
2. Find image etc in body, replace it
3. Update the email body here by
   • $this->getEvent()->set('mailer)->body = $yournewbody
   • or $this->getEvent()->set('body',$yournewbody)

PS : happy to help you to construct the core plugin :)

Created a plugin, thanks for info. :)

Shall I have this ticket closed? :)

Sure

Thanks for quick reply!

No …
The issue persist in LimeSurvey core

Waterlijn cp,nstruct a plugin but this plugin are not public or in LimeSurvey core plugins .

Yeah, but the idea is that it isn't a core problem. This would rather be solved in a plugin, so I did.
You can find it here. The source is as is and have fun with it. :-)
https://github.com/Xites/LS_mailSendMessage

Thanks for the public plugin :)

There is an old bug (14343) that causes problems when there is a subdirectory and relative images in the text.

You seems to don't use body of this event https://manual.limesurvey.org/BeforeTokenEmail , redo whole system (with HTML editor and so)
Not just the email body part.

Actually I didn't use the whole system for about 8 months now due to corona. But at the time it worked for me. I also wrote a plugin for DKIM, you might be interested.
https://github.com/Xites/LS_mailDKIM

LS_mailDKIM is really great !

About LS_mailSendMessage : there are other purpose than «just» include image ? Right ?

Sorry my bad, wrong source. This mailSendMessage was an experiment, not for publishing.
Her is the right one: https://github.com/Xites/LS_mailEmbedImages

Great plugin too !

Not same purpose than the original issue (include image with subdirectoy for URL)
But : maybe a better system

@Waterlijn : you're OK if i add your plugins in our wiki ?

Yes sure, no problem.

Don't find a good way …
Plugin seems the best solution

duplicate 14343: Included image are not send with email if subdirectory is set (3.X version)

So, can this ticket be closed :) ?

Yes please