Without knowing more about your libraries, e.g. file handling, can't be more specific
But what reference is from Notch's Metagun, he used this approach for his level in a ludumdare some years agohttps://github.com/alexnick/metagun/blob/master/src/com/mojang/metagun/level/Level.java
specifically look at pixels, the above Level.java class
Art.level.getRGB(xo * 31, yo * 23, 32, 24, pixels, 0, 32);
In this case level is a bufferedImage
as seen herehttps://github.com/alexnick/metagun/blob/master/src/com/mojang/metagun/Art.java
he uses the rgb value for determine wall or not wall. to make map
He then uses other factors for other variables as well, quite useful/neat.
Given a bufferedImage
load it up into the file
int pixels = new int[15*15]; //15*15 in your case, cause thats size of image?
then bufferedImage.getRGB( startX, startY, width, height, pixelsOut, offset, scansize);
you can also just get the x,y of something specific too
read up on http://docs.oracle.com/javase/7/docs/api/java/awt/image/BufferedImage.html
then your pixels has your data, and can just sweep thru it using x + y*w; // where w is 15 in your case.
and check if pixel == 0xFFFFFF or 0x000000 for white and black.