Error: Matrix dimensions must agree. How to solve?

This is my code for a uniform quantizer and I am getting an error for equation ''y''. %Project 2 %Quantization

clc; 
close all; 

org_image=imread('C:\Users\Aishwarya Kumar\Desktop\lena512.bmp');
subplot(2,2,1);
imshow(imread('lena512.bmp'));
title('Original Image');
org_image=double(org_image);

for x=5:7
  level= 2^x;
  lev1=256/(level); 
  [tk,rk]=tk_rk(lev1,level);
%Quantization 
 for i=1:256     
  for j=1:256
   for k=1:1:(level)

         if org_image(i,j)<tk(k+1) && org_image(i,j)>=tk(k)
             recon_image(i,j)=rk(k);
         end
     end 
    end
   end
  recon_image=double(recon_image);
  y=double(org_image-recon_image);

%MSE Calculation
MSE=sum(sum(power(y,2)))/(256^2);  

%PSNR Calculation
PSNR=10*log10((255^2)/MSE); 

recon_image=uint8(recon_image);

subplot(2,2,x-3);
imshow(recon_image);
title([num2str(level),' Levels Quantized Image']);
end

%Function definition
function[tk,rk]=tk_rk(level,lev1) 
tk(1)=0;
for a=2:1:(lev1+1)
tk(a)=tk(a-1)+level;
end
for a=1:1:(lev1)
rk(a)=tk(a)+level/2;
end
end

[KSSV]

Attach your image...or give the dimensions of it...

[Aishwarya Bangalore Kumar]

the dimensions of org_image is 512x512 but recon_image is 256x256

[KSSV]

This line:

y=double(org_image-recon_image);

Your org_image is 512*512 and recon_image is 256*256. How you think you can subtract them?