Hi Torben,
It's possible that the .tfw is not being honored by the Local Server. For example, in similar fashion, the .prj file of a shapefile is not honored.
If you start with a basemap that has the same spatial reference as your raster, add that basemap layer first to the map, then add your raster, that should help.
In a future release, you will add raster layers to your map in a more "direct read" fashion, so the Local Server will not be needed. The Local Server approach today is very very (did I say very?) limited with respect to dynamically adding ad hoc data sources to your map.
I hope this is helpful!