I've added a few images from the XenForo forum. There, you can easily adjust the size of the pictures added to the message. You add a link etc.
In our case, I could not see that there is a way to add the image to the Attachment section without uploading it and simply adjust it.
As an example I added it to our forum, but the picture is too big. You need to be able to scale it.
I used 5.3 RC 1 as version.