What are the possible ways to find spots with most concentrated contrast on an image?
Pass a sliding window of eg 100x100 pixels over the image stepped 1 pixel at a time in each dimension. Note the maximum and the minimum pixel value. The local contrast is now the local_max/local_min.
A more robust method might be to use a tapered window (eg Gaussian) and a sliding histogram, using eg the 10-percentile and 90-percentile to avoid salt and pepper noise corrupting results.
If (as stated in your update) you are interested in contrast of features larger than a pixel, downsample your image first.
-k