The internally packaged image files used for $Fill textures for map icons may be supplemented by adding JPG files to the folder~/Library/Application Support/Tinderbox/fill (create the folder if it doesn't already exist). The files should be in RGB colour, JPEG format. It is not necessary to re-start Tinderbox to us a newly added fill.
Custom fills should be referred to within Tinderbox by their filename minus extension (case-sensitive). Thus a custom fill 'Droplets.jpg' would be set by using a $Fill value of "Droplets" (though the actual name string seems to be case insensitive).