The .png files should work too.
If you’re using version 2.38, try this: put your new file in the icons directory. Then go to the MapPress settings screen and save the settings (without making any changes). That should force the plugin to refresh the icons list.
If that doesn’t work, feel free to send me a login and I’ll take a look for you. Please just tell me the name and directory of the file you expect to see.