The internally packaged image files used for $Fill textures for map icons and map backgrounds ($MapBackgroundFill) may be supplemented by adding RGB colour bitmap image files in JPG or PNG format to the fills folder ~/Library/Application Support/Tinderbox/fill (create the folder if it does not already exist). It is not necessary to re-start Tinderbox to use a newly added fill.
Custom fill files will be listed in the Fills pop-up menu.
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).