I really want to make a custom map that has a land mass that is shaped like an upside-down U, with a large mass of water filling a central basin, and several peninsular type spurs coming off the land mass at various points, and I have tried several times to do this but the game never seems to find the PNG files I create and allow me to use them.

Does anyone here have any pointers for an area that I may have got wrong? I have done my best to follow the instructions from several sources that tell me that a grayscale PNG file made to the right size should work.

Have a look at the release hints:

Release hints

There you can see how the folder structure should look like. You can also download an example and just try to replace the png in the example with your own.

Let me know if that helped.

Best Regards,
Tom