Display grayscale image as 3d plot

pic = Import @ "http://i.stack.imgur.com/RJs60.png"

ListPlot3D can work with an array of values too. The problem is that you have quadruplets there, RGB and alpha channel. Convert it:

ListPlot3D[
  Reverse @ ImageData @ RemoveAlphaChannel @ ColorConvert[pic, "Grayscale"],
  AxesLabel -> {"x", "y", "z"}
]  

Another nice 3D-ish plotting command is ReliefPlot. Modelling the preprocessing after Kuba's answer, this would be:

pic = Import@"http://i.stack.imgur.com/RJs60.png";
imdData = Reverse@ImageData@ RemoveAlphaChannel@ColorConvert[pic, "Grayscale"];
ReliefPlot[imdData]