This only concerns the desktop version of Quest.

I just learned that res:// is the directory that Quest.exe is in.

In the directory containing Quest.exe, there is an "images" directory which contains this file: "ui-bg_gloss-wave_55_5c9ccc_500x100.png"

msg ("<img src=\"res://local/images/ui-bg_gloss-wave_55_5c9ccc_500x100.png\" />")

And quest:// is the game directory.