How to insert pixel values?

First of all i would like to thank you for your quick reply.I think that in your example code you define an 1 dimensional array and not a 2 dimensional.To define a 2 dimensional i think that you have to write it like this(if i am right):

type PIXEL_TYPE is array (natural range 0 to 19,natural range 0 to 19) of std_logic_vector(19 downto 0);

signal pixel_array: PIXEL_TYPE;

I am using FPGA.My algorithm is about to make a hardware implementation of a neuron(the main part of a neural network).

I would like to insert a window of the whole image each time.For example if the whole image is 400x400 pixels i would like to insert to the neuron a 20x20 window.