Hi Dave,
You need to import your tracings as either custom polygons or custom paths, depending on whether you created lines or polygons within GE. You will not be importing the image into RM, as such.
Go to your user map, select the custom paths/polys group and click the import custom... button and choose your KMZ file. You might want to create a KMZ file separately for paths and polys.
Once your lines are in the user map, you can edit them - width, colour, fill and so forth - and export them to whatever format you need. If you are creating a map for a Lowrance device that supports raster backgrounds (HDS and later, Elite HDI) then you might want to create this raster background using the IMC directly and your image overlay KML file. See
this thread for more on how to do this.
Matt