I cannot draw a decent square or circle to save my life, so when trying to draw a cutout of an image that is in the shape of a square or circle, or oval shape it ends up being horribly ugly.
Save the project as a ZIP file, unpack the ZIP and you'll see a list of PNG files representing individual regions (_seg_*.png) and outlines (_org_*.png). You can edit these images in your favorite image editor and pack them back to a ZIP and open it in Monster Mash. Note that the outlines must correspond to the regions. Therefore usually the best approach is to edit a region image and create an outline image from it manually (e.g. using a lasso selection tool, shrinking the selection, and removing the interior of the region so that only a thin outline remains). Also beware that the described approach may cause Monster Mash to crash, especially if the gray interconnection outlines are more complicated.