Nope, and this is the point !
So, you want to use an image format that you haven't a clue how to read.
It's obvious on Processing
that someone who DID understand the image format was able to write a function to extract data from it.
but on Arduino I can't find any hint about this
Because the Arduino has no place to display an image. Why should there be functions and methods to deal with image data when it does not have image display capabilities.
Get yourself over to Google (or whatever search engine you like) and learn how to access the image data in a png file.