Feature Extraction of Image by using MFCC

Good day.

I am currently doing an assignment regarding feature extraction of image by using MFCC. Based on what I found, I have to convert my 2D image into 1D signal and I have done this step (following code):

a = rgb2gray(imread('filename.jpg')); %2D grayscale image

signal = a(:) %1D signal formed by concatenation of all rows

However, i am facing problem of extracting feature from the 1D signal.

Does anyone have any source code regarding this?