simplify PNG explanation, but keep the luminance in, since it has scientific applications.

Author: Florian Goth

_episodes/02-image-basics.md

@@ -390,6 +390,9 @@ plt.imshow(three_colours,cmap=plt.cm.gray)
 
 Above we have exactly the same underying data matrix, but in greyscale.
 Zero maps to black, 255 maps to white, and 128 maps to medium grey.
+Here we only have a single channel in the data and utilize a grayscale color map
+to represent the luminance, or intensity of the data and correspondingly
+this channel is referred to as the luminance channel.
 
 ## Even More Colours
 
@@ -931,8 +934,7 @@ you may wish to use a compressed image format to speed up file transfer time.
 ## PNG
 
 PNG images are well suited for storing diagrams. It uses a lossless compression and is hence often used 
-in web applications for non-photographic images. The format is able to store RGB, RGBA, plain luminance data, as well
-as images with a palette. Image data is stored row-wise and then, per row, a simple filter, like taking the difference of adjacent pixels, can be applied to
+in web applications for non-photographic images. The format is able to store RGB and plain luminance data, among others. Image data is stored row-wise and then, per row, a simple filter, like taking the difference of adjacent pixels, can be applied to
 increase the compressability of the data. The filtered data is then compressed in the next step and written out to the disk.
 
 ## TIFF