The 'fill' folder name is case sensitive' and it found at:
~/Library/Application Support/Tinderbox/fill/
The folder holds any custom fill files files (see below) used for map view backgrounds or map note icons.
The Fill folder and its subfolders may contain aliases of image files as well as the images themselves.
To make it easier for the general Mac user to access Tinderbox's support folders, the Help menu's 'Reveal Support Folder in Finder' option will open the folder in a new Finder window.