>>2
Why do you care about extraneous elements in the web page when the PNG itself is 13,628 bytes larger than it could be?
christopher@chrislaptop$ ls -o blackcat.png
-rw-r--r-- 1 christopher 52310 Mar 21 20:37 blackcat.png
christopher@chrislaptop$ optipng -o7 ./blackcat.png && pngout ./blackcat.png
OptiPNG 0.6.1: Advanced PNG optimizer.
Copyright (C) 2001-2008 Cosmin Truta.
** Processing: ./blackcat.png
300x861 pixels, 8 bits/pixel, 256 colors in palette
Reducing image to 4 bits/pixel, 16 colors in palette
Input IDAT size = 51364 bytes
Input file size = 52310 bytes
Trying:
zc = 9 zm = 9 zs = 0 f = 0 IDAT size = 40758
zc = 9 zm = 8 zs = 0 f = 0 IDAT too big
zc = 8 zm = 9 zs = 0 f = 0 IDAT too big
zc = 8 zm = 8 zs = 0 f = 0 IDAT too big
zc = 7 zm = 9 zs = 0 f = 0 IDAT too big
zc = 7 zm = 8 zs = 0 f = 0 IDAT too big
zc = 6 zm = 9 zs = 0 f = 0 IDAT too big
zc = 6 zm = 8 zs = 0 f = 0 IDAT too big
zc = 5 zm = 9 zs = 0 f = 0 IDAT too big
zc = 5 zm = 8 zs = 0 f = 0 IDAT too big
zc = 4 zm = 9 zs = 0 f = 0 IDAT too big
zc = 4 zm = 8 zs = 0 f = 0 IDAT too big
zc = 3 zm = 9 zs = 0 f = 0 IDAT too big
zc = 3 zm = 8 zs = 0 f = 0 IDAT too big
zc = 2 zm = 9 zs = 0 f = 0 IDAT too big
zc = 2 zm = 8 zs = 0 f = 0 IDAT too big
zc = 1 zm = 9 zs = 0 f = 0 IDAT too big
zc = 1 zm = 8 zs = 0 f = 0 IDAT too big
zc = 9 zm = 9 zs = 1 f = 0 IDAT size = 39871
zc = 9 zm = 8 zs = 1 f = 0 IDAT size = 39822
[...some output omitted...]
zc = 1 zm = 8 zs = 2 f = 5 IDAT too big
zc = 9 zm = 9 zs = 3 f = 5 IDAT too big
zc = 9 zm = 8 zs = 3 f = 5 IDAT too big
Selecting parameters:
zc = 9 zm = 8 zs = 1 f = 0 IDAT size = 39822
Output IDAT size = 39822 bytes (11542 bytes decrease)
Output file size = 40048 bytes (12262 bytes = 23.44% decrease)
In: 40048 bytes ./blackcat.png /c3 /f0 /d4
Out: 38682 bytes ./blackcat.png /c3 /f0 /d4, 16 colors
Chg: -1366 bytes ( 96% of original)