Contouring an Image with ListContourPlot

Question

I'm trying to take an image that consists of several noisy ellipses and use ListContourPlot to generate a contour of the image. However, I'm just getting a single dense line. If I binarize the image I get the correct result, but I lose the detail needed for the contour to be good. However, if I use Posterization with a value of two, it also doesn't work. What is Binarize doing that Posterize isn't, and why does ListContourPlot work with one but not the other?

Sidenotes: I've looked at the image data and the background of the image even before Posterization is mostly true black.

Contours also work with DistanceTransform.

Code:

FN = DialogInput[{filename = temp}, 
   Column[{"Browse for your file:", 
     InputField[Dynamic[filename], String, 
      FieldHint -> "Enter your file name"], 
     FileNameSetter[Dynamic[filename], "Open", 
      Method -> "Preemptive"], 
     DefaultButton[DialogReturn[filename]]}]];
img = Import[FN];
img = ColorConvert[img, "Grayscale"];
(*img = ChanVeseBinarize[img]*)
img = ImageEffect[img, {"Posterization", 2}]
pic2 = ListContourPlot[ImageData[img, DataReversed -> True], 
   ContourShading -> None, Frame -> False, ContourStyle -> Yellow];

Show[img, pic2]

Image In:

Image Output:

Answer 1

With chuys hint:

img = Import["https://i.sstatic.net/vkvMu.png"]

imggray = ColorConvert[RemoveAlphaChannel@img, "Grayscale"]

colTable = {Black, Blue, Green, Yellow, Red}; 

contour = ListContourPlot[ImageData[imggray, DataReversed -> True], 
            ColorFunction -> (Blend[colTable, #] &), AspectRatio -> Automatic, 
            ImageSize -> Medium]