A workaround is to in Mapsource select to print to pdf (you need to have Adobe Acrobat installed) and select a LARGE paper size, i.e. A0. Select a usable zoom level in Mapsource.
Then open the generated pdf with Acrobat and export to jpg. Works ok but needs some trial and error of course.
And there is always the simple way to prinr screen from Mapsource...