How to assign an image to another with a black frame?

I used the code given below to assign another picture to a black filled picture. The given picture has to fit into the black background at the center so that it looks as if it is framed.

But the result is:

I'd like the pic I loaded, (that is j) inside the box. I tried a lot! Please help! Many thanks in advance! :)

j=imread('1.jpg');
[na, nb, nc]=size(j);
b=40;
j_big=zeros(na+2*b, nb+2*b, nc);
j_big(b+1:b+na, b+1:b+nb, :)=j;
imshow(j_big);

Answer 1

[Image Analyst]

Why not just use padarray()?

[Edwin Joy]

Oh okay. Thank you!

But I still don't understand why I just get some pixels scattered around the image when I use the code mentioned. I even tried to use loops and copy pixel by pixel to find out the error but it's still giving me the same result.

EDIT:

Yay! I got it. Well it appeared when I changed the array to unsigned int8.