LEVEL: VERY EASY
Note: The image was not generated with any steganography tool.
Note2: In this case, the message can be extracted without coding.
Hint 1:GIMP is your friend
Hint 1.5:Use the Color>Components>Decompose to get the image components separated.
Hint 2:PGM format saves grey scale images as raw data
Hint 3:The image is pretty blue, isn't it?. Take a look to the colour components