"I'm looking for a matlab function that let me analyse different regions of an image depending of the peaks of its histogram."
The short answer to this question is that you cannot.
Histograms tell you the number of pixels for a particular intensity value and not their location.
For example, Consider these 2 images of size [3, 3].
A = [3 5 7 ; 4 1 3 ; 4 3 5];
B = [1 3 4 ; 5 4 3 ; 7 3 5];
If you take the histogram of these two images, you would get the same figure and values and you wont be able to tell which image the histogram belongs to or where are the pixels with intensity value "5" located.
"I take picture of patients that have skin lesions and I process their images along time, to see through the mean of their image how they evolve."
This problem is usually solved by segmenting the lesions from the image and calculating its area. Based on the increase or decrease in the area, you can see if the patient is getting better or not.
