skew map on exportation

This sounds like it's a projection issue.

Map co-ordinates are usually measured in 'degrees' of latitude (y / vertical) and longitude (x / horizontal). All of OpenStreetMap's data is stored in co-ordinates like this. Have a look at a spherical globe to see how big a degree is.

If you simply draw your map such that each degree is represented by a constant distance - say, 1cm on the map for 1 degree of the world - then the map will end up squished in exactly the way you describe.

A 'projection' determines how much space on your screen/print-out each degree takes up. To avoid this squishing effect, a simple projection - such as the one used on most mapping sites, technically known as 'Spherical Mercator' (or 'Web Mercator') - gives more space for each degree of latitude (y) as you get further away from the equator. (In this projection, each degree of longitude takes up the same space.)

You therefore need to tell Ghost Town to use a different projection. You'll need to ask on the forums for that product how to do that.

(For a more light-hearted look at projections, see this xkcd cartoon.)