Similar comment to above, If you want to convert the equidistant projection to say a transverse mercator where latitude and longitude grid is rectangular then there might be some clever web application somewhere.
EDIT: Sorry, failed to notice your MacOS system. No gmic plugin for you.
You can get a rectangular image using the gimp_gmic_qt plugin http://www.gmic.eu filter square-to-circle / circle-to-square looks like this:
EDIT: Sorry, failed to notice your MacOS system. No gmic plugin for you.
You can get a rectangular image using the gimp_gmic_qt plugin http://www.gmic.eu filter square-to-circle / circle-to-square looks like this: