How about starting with i.e. @image(...) tag, where content is base64 encoded image data? This way all remains in single, parsable .taskpaper file.
Editor could have two modes:
1. basic, where @image(...) is shown as text (with tag content truncated perhaps, not to include tons of meaningless characters)
2. expanded, where @image() is rendered as an image (surprise).
I use TaskPaper to write software specification for my customers, and would love to embed a small image every now and then.