I am not sure what you are trying to accomplish but I don't think you will need the Drawables class. You can use the
1.
This does not draw the Image on the Drawables but it draws the drawables on your image. And because your Drawables collection is empty nothing will happen.
2.
This returns an image that you seem to be ignoring. This image is the result of your Mosaic call.
3.
This writes a png image and not a gif.
Composite method to draw one image on top of another image. You should give that a try. And below are some other comments on your current code.1.
var bgDraw = new Drawables(); bgDraw.Draw(bgImage);
2.
images.Mosaic();
3.
images.Write(frameMs, MagickFormat.Png);