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I don't understand what I have to do. I examined in 800x zoom with GIMP, but it doesn't give me any hint about the answer.
Could someone give me some hints on how to proceed? Where do I have to focus on?

Like the text in the challenge says the pixels represent a series of bytes. So try converting the pixels to bytes and see if you can find a meaning for them.

The hint is in the problem description: "these pixels represent a series of bytes". Pixels, not image. There's something about each pixel that varies. Work out what one pixel is in this image.

laz0r wrote:There's something about each pixel that varies.

I looked at every pixel. All of them have different values of green, and noone has some red or blue in it.
Most have only numbers, but others have also letters in the green part of the Hex code. Are the letters important?

You're on the right track. The letters are exactly as important as the digits. Ignore the fact that it's hex; try putting it into decimal and see if you spot something (a very basic something; don't stare too hard at the numbers, because they're not really what you're looking for).
The picture itself is quite suggestive, too; if you received this as a picture, ignoring the fact that the image is divided into pixels, are there any structures you can see in it? (sorry if I'm not being very helpful; it's quite hard to hint about the topic that this challenge is based on without revealing the whole thing!)
If you want further hints, you can private message me using the Private Messages link at the top of the page, and going to New Post.

Thank you laz0r, i think I'm solving it soon