Hi Maximilian,
one simple idea would be to overlap your image with its point density:
img = Binarize[ < your original image > ];
dims = ImageDimensions[img];
pos1 = ImageValuePositions[img, 1];
cimg = SmoothDensityHistogram[pos1, 15, Frame -> False,
ColorFunction -> "Rainbow", ImageSize -> dims, PlotPoints -> 100,
PlotRange -> Full];
ImageMultiply[cimg, img]
Does that help? It runs fast, at least ...
