Believe it or not, you cannot reply to a Quickpost with an image. In the modern sharing world that is not a default behavior. Vote to restore (yes restore, it was stripped from the editor) the Quick Post Reply with Image.
This is a bug, not a feature request. I've logged it as:
The work-around in 10.1 is to edit the Activity Story Stream widget's callback-geteditor.vm file and change line 16 from:
The issue is that the parent content type's filtering rules are processed (status message) instead of the reply's content type (comment, in this case). The content filtering rules are being applied correctly, but status messages don't allow embedding files by default and comments do.