Based on the description, you may have determined that we are dealing with an autostereogram, better known as a magic eye puzzle. As such, theoretically you can just stare at the picture with great intensity until the flag reveals itself to you. If you did manage to solve this challenge only with your eyes then you are amazing. Discerning simple shapes are difficult, let alone a short sentence.
If you are a mere mortal, an alternative way to solve this challenge is to use any number of the online sterogram solving tools that exist (this one’s pretty good). Alternatively you can open up GIMP and put the image on two layers. Then set the blending mode of the top layer to ‘difference’ and drag the top layer along the horizontal axis until you get something like this:
Regardless of what solution you use, the shape of the letters should bear resemblance to the original image, shown below.