Expand the Tux ECB image example

While cryptographers know the image and can imagine the original and
how it looks in ECB, many who start their path cannot imagine that process.
This commit updates the images to show the original (from wikipedia - converted
to png), the ECB-encrypted, and one encrypted with CTR (mentioned as GCM to
simplify the story, as CTR is not yet covered).

Signed-off-by: Nikos Mavrogiannopoulos <nikos.mavrogiannopoulos@assaabloy.com>

secure_software_development_fundamentals.md

@@ -4499,7 +4499,9 @@ Many symmetric key algorithms, including AES, are what is called *block algorith
 
 The ECB mode is basically a debug or test mode for testing cryptographic algorithms. In ECB mode, the same block of data will produce the same encryption result. This is disastrous for an encryption algorithm, because it reveals far too much about the data that is supposed to be encrypted. A great illustration of this is the so-called “ECB Penguin” image; this image is encrypted using an ECB mode. Encrypted images should appear as random noise, but because ECB mode is used, in the ECB Penguin the image of Tux the Penguin is clearly visible.
 
-![The ECB Penguin: A dark encrypted image that clearly shows the Linux mascot, Tux the Penguin](images/ecb_penguin.png)
+![The original Penguin](images/penguin.png)
+![The ECB Penguin: A dark ECB-encrypted image that clearly shows the Linux mascot, Tux the Penguin](images/ecb_penguin.png)
+![The GCM Penguin: An encrypted image that uses Galois/Counter mode (GCM) excluding the tag](images/ctr_penguin.png)
 
 The ECB Penguin, by Filippo Valsorda, retrieved from [filippo.io](https://blog.filippo.io/the-ecb-penguin/). Licensed under [CC BY-SA 4.0 International](https://creativecommons.org/licenses/by/4.0/legalcode). This image was inspired by the original lower-resolution ECB Penguin image by Wikipedia User: Lunkwill. Source “The ECB Penguin” (2013-11-10). Based on the Tux the penguin official Linux mascot created by Larry Ewing in 1996