Right, there's no way to upload the type of map TF uses (Raster/Image) to your GPS. The GPS uses Vector maps, of which Garmin's Mapsource is the only source.
It is possible to make your own vector maps and upload them, and there is some limited 3rd party software on how to do this, but it's not easy and datasets are very sparse.
Pretty much if you want maps on the unit, you use mapsource.