Once you've figured out the closest X, Y point from your depthArray, you're going to want to set up another double-for-loop to iterate through the pixels of your image. For every one of your image pixels, you're going to want to calculate it's distance from the closest X,Y point and then draw a rectangle using the same color as that pixel but adust the width and height of rectangle according to the calculated distance. Here's the flashlight example which shows adjusting the brightness of a pixel according to the distance from the mouseX, mouseY point.
Basically substitute closestX, closestY for mouseX, mouseY.
// Calculate the distance of this point in your image to the closestX and closestY points float distance = dist(x,y,closestX,closestY);
// Calculate the size of the rectangle based on the distance from the closestX, closestY point // As the distance gets larger, the pixel gets smaller // You will probably have to fiddle with these numbers to get the effect you want float rectSize = map(dist, 0, 100, 10, 1);
// Use the color of the pixel to color the rectangle fill(pixels[loc]);
// Draw the rectangle rect(x, y, rectSize, rectSize);