I'm using the readJPEG() function from the R jpeg package to read in the JPEG image. Alternatively, you can use the readPNG() function from the R png package to read in a PNG image. If you read the help file of readJPEG(), it says it "reads an image from a JPEG file/content into a raster array". OK, first off what do we mean by raster? You can think of a raster image as a rectangular grid of pixels. In the help file for the readJPEG() function, it states "most common files decompress into RGB channels (3 channels)" where RGB stands for red, green and blue.
So, this means for each pixel in the original image, we should expect 3 values (one for red, green and blue). These values range between 0 and 1 and can be thought of the intensity of each color in the image. Below, I'm showing the intensity values from the three channels colored on the same scale (top row) and colored in the red, green and blue scales (bottom row).
Next, we sample points proportional to the (powered) intensity level, add a bit of random noise and then plot the sample points along the x and y axis. In the picture below, you can really see how the power is very important to control the contrast levels.