(*Adapted by Rafael Luís: rafaelluis@netmadeira.com
updated to Mathematica v.10 by Ross Moore *)

DensityPlot[
  -Length[
     FixedPointList[ #^2 + (x + I y) &, x + I y, 50,
       SameTest->(Abs[#2] > 2.0 &) ]],
  {x, -2.01, 0.7}, {y, -1.1, 1.1},
   PlotPoints -> 225, Mesh -> False, Frame -> False
,ColorFunction->GrayLevel,
   AspectRatio -> Automatic ]