Hello, I'm having the same issue.
I can copy an image from another web browser window and paste into the compose window. The image shows up. Looking at the page source, it creates an <img> tag with the data in the src attribute. Here's the original message formatted.
Code:
--=_997dd3f9-cfe1-4e99-8300-ed321f00246c
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable
<html><head><style type=3D'text/css'>p { margin: 0; }</style></head><body><=
div style=3D'font-family: Arial; font-size: 12pt; color: #000000'><span><br=
></span><img src=3D"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAY0AAACgC=
AYAAADw4jCNAAAC7mlDQ1BJQ0MgUHJvZmlsZQAAeAGFVM9rE0EU/jZuqdAiCFprDrJ4kCJJWato=
RdQ2/RFiawzbH7ZFkGQzSdZuNuvuJrWliOTi0SreRe2hB/+AHnrwZC9KhVpFKN6rKGKhFy3xzW5=
...
a9HRbXl6eFRf8xz/+cVZF0UWEgQgDEQYiDLz/MfD/ARTVPcY4zN+ZAAAAAElFTkSuQmCC" alt=
=3D""><br><br><span id=3D"6db0ca58-6b6f-4ece-81d6-8e9f08449efd"><br><br><sp= If you look at the message in the viewer, it just says <img alt="">. I think those "3D" characters are what is the problem.