To modify the output for a supported variant, you can modify the mapit files, try them on some histories you get from the judge and send me the new files.
For a new variant, David Norman usually provides automatically generated maps for new variants played on USTR, see some help pages on his site
Chris Jones wrote a nice guide about making maps for new variants or enhancing existing maps.
Here is another guide by Juho Snellman with more information about how to use a PS viewer to define the coordinates and a vector drawing program.
If you have a bitmap for your variant, you may want to convert it to postscript with a program like autotrace instead of starting from nothing. Try different option to adjust the rendered details.