A popular way to animate some characters is to use video referencing for accuracy. If mosketch would allow users to import and scrub through a transparent video in the background, it would help LOADS!
Here is an example of this usage in Blender:
While the user prefers the other method, the video could be simply toggled in mosketch if needed.
reference planes (with images or videos) are indeed really helpful. We will see what we can do but to be honest, it will not be released that soon unfortunately.
All the best and thanks for the feature request.