[caoimhin]

Yes the code would go into your core_mapstyles.xml file and the dds images go into the images\maps folder within eq2maps.

Beware that manually changing the core_mapstyles.xml file will cause the eq2maps update to see it as incorrect and replace it any time you run the updater. You can either:
Reset it after each time you run the updater,
Not run the updater or
Lock the file by making it read only until such time as you want it updated.