Share on Facebook button with image from within an email

Question

I send an email to users by using mailgun. The email consists of some text and multiple images (integrated like )

I would like to add a Share button which leads the user to facebook and automatically adds the image(s) to the post. also it would be great to add a link to a website.

As I understand, facebook sharing needs a website to crawl for Open Graph tags but the email is no website to crawl, right? Also I've read that facebook discontinued the sharer.php which seem to accept parameters. I tested the sharer.php and it still seems to work. Does anyone know how to add one or more image links to this url? I guess I have to format the link somehow. Will this work in the future or will they remove it? https://www.facebook.com/sharer.php?u=123.abc.com&t=Test

Any idea how I can let my users share the images I sent them via mail other than having them download the images and upload it manually to facebook?

Thank you so much in advance, Robert

I tested https://www.facebook.com/sharer.php?u=123.abc.com&t=Test&picture={a link to the picture} but even when I use urlencoder.org to encode my url, I can't see the image. I guess this tool is deprecated but how can I add a share button nowadays?