And while this works for embedded images, it doesn’t work for image previews from web links. I imagine it’s the .internal-embed part of the css code that makes it work for embedded images, I’ve been unable to find any guidance on how to modify the css snippet to work for web link previews.
In a note where I have the same image first as an internal embedded image, and then as a web link:
For others looking at this solution, .markdown-source-view.mod-cm6 .cm-content > img,
is in addition to the other .markdown lines, so this is the full css snippet that now applies the border/shadow to both external and internal embedded images: