Set default image when posting with Postie plugin

Good morning,

we have a question as to how to set a default image/template for the Postie plugin.
Currently if we make a post without a picture it posts the default grey placeholder picture which we want to replace with another image.

We tried changing the image template in the plugin settings in the “image” tab.
We uploaded the picture we want to use to the media library and used the internal URL as the ‘a href’. The “Preferred text type” is set to “plain” aswell, but we still had no success.

Now we’re wondering if it’s even possible to set a default image for Postie.
We basically want to have this image as the default everytime a post is made by someone without a picture.

We would be very grateful if you could help us out.
Thanks in advance!