shr and eww can't work in Emacs 23. Regardless of other incompatibilities, they require Emacs to have been compiled with libxml2 support, and that ability came in 24.1
If you want to try eww, you should install Emacs 24.4 or later ("later" currently meaning compiling from the source repository).
Note that eww is built into these versions (24.4+) of Emacs, ...
shr.el has a (defvar shr-inhibit-images nil), and a
(defcustom shr-blocked-images nil
"Images that have URLs matching this regexp will be blocked."
:type '(choice (const nil) regexp))
It seems like (setq shr-inhibit-images t) stops the web requests when I view HTML emails.
Note that it turns off image display for eww ...
If you want to generally change/add the rendering of nodes of a certain tag type you just add an association from the tag as symbol to the rendering function in the list shr-external-rendering-functions.
Example: Render <em> nodes using a self defined function my-tag-em:
(add-to-list 'shr-external-rendering-functions '(em . my-tag-em))
I have tested ...