My freebie thresholding function works well:
1. The Mona Lisa (mona.pgm) (Threshold values are 16 to 240 in increments of 16)







2. Group Letter (K) (letterK.pgm)


(From left to right: threshed at 128, contrasted, original's histogram. Contrasted version looks about the same because the noise spans the whole domain)

3. Example Tractor Picture


(From left to right: threshed at 128, contrasted, original's histogram)