Specify format (P1), width, height and 0 (white) or 1 (black) for every pixel.
P1 10 10
0 0 0 0 0 0 0 0 0 0
0 0 1 0 0 0 0 1 0 0
0 1 1 1 0 0 1 1 1 0
0 0 1 0 0 0 0 1 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0 1 0
0 0 1 0 0 0 0 1 0 0
0 0 0 1 1 1 1 0 0 0
0 0 0 0 0 0 0 0 0 0
Save as image.pbm.
Convert to PNG with convert image.pbm image.png.