Note head as a custom image chosen according to note duration

[Jean Faure]

Hi. The code below is a copy&paste, I do not quite understand what it does. Can I modify it so that an image is chosen according to the following note's duration? Like custom4.eps, custom8.eps?

customHead = {
 \once \override NoteHead  #'stencil = #ly:text-interface::print
 \once \override NoteHead #'text =
   \markup {
     \general-align #Y #CENTER {
       \epsfile #X #2 #"./custom.eps"
     }
   }
 }

{ e'4 f' \customHead g'4 }