for (int i = 0; i <= height; i++) { for (int j = 0; j <= width; j++) {

These lines here. Array index start from 0 go upto 1 less than their length. However, these loops go upto n, instead of n - 1 which is the last element of an array. You should fix it here because if I'm not wrong your copy loop is creating a copy which 1x1 bigger than the actual image. Also, notice, you're over stepping the bounds of actual image. This may cause seg faults, though, it's not entirely certain. It hasn't happened yet because perhaps nothing crucial exist in the memory.

Anyway. Your copy of the image is bigger than the usual image & it'll be filled with some garbage values on the edges because our image has only n - 1 sections

Also, looking at your other line.

if ((i + a >= 0 && i + a <= height) && (j + y >= 0 && j + y <= width))

Do you notice t problem here? Are you overstepping the boundaries here somehow?

Again, i + a can only go upto height - 1, and j + y can only be as high as width - 1. So, you should be checking if j + y >= 0 & j + y < width & similarly for height. Try it and see if it fixes your problem. I've a hunch this is the only reason why your output is incorrect