This doesn't support images as far as I can tell
Trix also supports "content attachments" (currently undocumented) which allow embedding arbitrary HTML as attachments in the document. Basecamp 3 uses this feature to embed @mentions with avatars, and various oEmbed types, in messages and comments.
I'm currently using CKEDITOR in a Rails app and inserting HTML "templates." CKEDITOR renders the HTML and allows the end-user (or content creator) to modify the HTML while it is rendered inside the contenteditable text area.