link scatter plot to the image

I have an image(500*500) and I calculated two images from the original image, those derived images(a and b) contain the calculated values, which directly corresponding to the the pixels of original image. I plotted the scatter plot between a and b(it includes the 500*500 points). I want to link the scatter plot to the image. then i can map the areas which are having pixels grater than a...etc. and i want to calculate another variable called c from a and b for each pixel. the c is ranges from 1 to 0. 0 indicates the dangerous areas and 1 indicates the no risk areas. i want to use graduated colours in between 0 and 1 and plot the map.how can i do it?