"image file is not of type 15"

gistfile1.txt

/* [wxMaxima batch file version 1] [ DO NOT EDIT BY HAND! ]*/
/* [ Created with wxMaxima version 0.8.7 ] */

/* [wxMaxima: input   start ] */
kill(all);
/* [wxMaxima: input   end   ] */

/* [wxMaxima: input   start ] */
chull(z, a, b) := (a+b)*z - (a*b);
/* [wxMaxima: input   end   ] */

/* [wxMaxima: input   start ] */
lbound: -2;
/* [wxMaxima: input   end   ] */

/* [wxMaxima: input   start ] */
axis_max: 8;
/* [wxMaxima: input   end   ] */

/* [wxMaxima: input   start ] */
axis_min: -4;
/* [wxMaxima: input   end   ] */

/* [wxMaxima: input   start ] */
zmin(a, b) := if a >= 0  then a else if  b <= 0 then b else 0;
/* [wxMaxima: input   end   ] */

/* [wxMaxima: input   start ] */
zmax(a, b) := if a^2 >= b^2 then a else b;
/* [wxMaxima: input   end   ] */

/* [wxMaxima: input   start ] */
with_slider_draw(
  t, makelist(i, i, 0, 4),
  xaxis=true, yaxis=true,
  xrange=[axis_min, axis_max], yrange=[axis_min, axis_max],
  color = red,
  explicit(x^2, x, axis_min, axis_max),
  color=blue,
  implicit(x=lbound, x, axis_min, axis_max, y, axis_min, axis_max),
  implicit(x=t, x, axis_min, axis_max, y, axis_min, axis_max),
  color=green,
  explicit(chull(x, lbound, t), x, axis_min, axis_max),
  

  color=purple,
  explicit(median(
    [x^2, chull(x, lbound, t), zmax(a, b)]),
    x, axis_min, axis_max)
  
  
)$
/* [wxMaxima: input   end   ] */

/* Maxima can't load/batch files which end with a comment! */
"Created with wxMaxima"$