Exercise 1: Why the ECB Penguin Stays a Penguin — Possible Solution
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A bitmap image is made up of many flat, uniformly-colored regions --
the sky, the penguin's white belly, its black back -- each of which
corresponds to long runs of IDENTICAL or near-identical pixel data
when the image is split into fixed-size blocks. Under ECB mode, each
of those blocks is encrypted completely independently, using the same
key and the same algorithm every time.

Because ECB's defining property is that identical plaintext blocks
always produce identical ciphertext blocks, every block belonging to
one of those flat colored regions still encrypts to the SAME
ciphertext value as every other block from that same region. The
region's boundaries -- where the pattern changes from one repeated
block value to another -- are still visible in the ciphertext, in
exactly the same positions they occupied in the plaintext. Nothing
about ECB mode scrambles the LAYOUT of the image, only the specific
byte values within each block.

Connection to Chapter 2's monoalphabetic substitution:

This is exactly the same failure mode as monoalphabetic substitution,
just operating on 128-bit blocks instead of single letters. A
monoalphabetic cipher maps every occurrence of a given plaintext
LETTER to the same ciphertext letter every time, which is what let
frequency analysis recover the mapping in Chapter 2. ECB mode maps
every occurrence of a given plaintext BLOCK to the same ciphertext
block every time -- a fixed mapping at a larger granularity, but the
identical underlying flaw: any repetition or structure in the
plaintext survives, unchanged in shape, into the ciphertext.

WHY THIS WORKS AS AN ANSWER
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The chapter states this connection directly ("structurally the exact
same weakness Chapter 2 spent an entire chapter exploiting... a fixed
mapping... always leaks the structure of whatever it's applied to").
This answer makes that connection concrete by tracing exactly how a
flat-colored image region becomes a sequence of identical plaintext
blocks, and why AES's own strength as a cipher (Chapter 5) is
completely irrelevant to this specific failure -- the weakness is
entirely in the MODE, not the cipher.