I googled "getpixel" on Arduino but several posts was about "TFTLCD library" and SD library. It doesn't match with my project which is to load an image on a Rainbowduino + 8x8 RGB.
I didn't say that you could use the same function that you use in Processing. That function accesses specific data in a specific format, and returns the data that it finds. You have not defined what data you will call your GetPixel() function for, nor have you defined the format of that data. Once you do, writing GetPixel() will be trivial.