Aha: after patching Elfeed to render after buffer display (which 
seems to fix the display of raster images), I actually did find 
behaviour that I believe should be addressed in shr, not in 
Elfeed; let me know what you think.

If an image is a SVG, `shr-put-image` will not respect 
`shr-max-image-proportion`, because it will pass it directly to 
`create-image`, instead of resizing it:
#+begin_src elisp
;; in `shr-put-image`
((eq content-type 'image/svg+xml)
         (when (image-type-available-p 'svg)
   (create-image data 'svg t :ascent shr-image-ascent)))
(t
 (ignore-errors (shr-rescale-image data content-type
                                           (plist-get flags 
                                           :width)
                                           (plist-get flags 
                                           :height)))))))
#+end_src

As I understand it, this decision was likely made because the 
dimensions of SVGs is weird; they aren't inherently required to 
have a size, but the SVG itself can define a viewbox with 
accompanying width and height (full disclosure: I have no idea 
what I'm talking about).

Using `shr-rescale-image` instead seems to do the trick (and 
respects SVGs that should display smaller than 
`shr-max-image-proportion`), but please do let me know if I'm 
ignoring an edge case or there's a better way to address this. 

#+begin_src elisp
(setq shr-put-image-function (lambda (spec alt &optional flags)
  (if (display-graphic-p)
      (let* ((size (cdr (assq 'size flags)))
	     (data (if (consp spec)
		       (car spec)
		     spec))
	     (content-type (and (consp spec)
				(cadr spec)))
	     (start (point))
	     (image (cond
		     ((eq size 'original)
		      (create-image data nil t :ascent 100
				    :format content-type))
             ;; BEGIN fix
		     ((eq content-type 'image/svg+xml)
                      (when (image-type-available-p 'svg)
		                ; (create-image data 'svg t :ascent 100)))
		                (shr-rescale-image data 'svg (plist-get 
		                flags :width) (plist-get flags :height))))
             ;; END fix
		     ((eq size 'full)
		      (ignore-errors
			(shr-rescale-image data content-type
                                           (plist-get flags 
                                           :width)
                                           (plist-get flags 
                                           :height))))
		     (t
		      (ignore-errors
			(shr-rescale-image data content-type
                                           (plist-get flags 
                                           :width)
                                           (plist-get flags 
                                           :height)))))))
        (when image
	  ;; When inserting big-ish pictures, put them at the
	  ;; beginning of the line.
	  (when (and (> (current-column) 0)
		     (> (car (image-size image t)) 400))
	    (insert "\n"))
          (let ((image-pos (point)))
	    (if (eq size 'original)
	        (insert-sliced-image image (or alt "*") nil 20 1)
	      (insert-image image (or alt "*")))
	    (put-text-property start (point) 'image-size size)
	    (when (and shr-image-animate
                       (cdr (image-multi-frame-p image)))
              (image-animate image nil 60 image-pos))))
	image)
    (insert (or alt "")))))
#+end_src elisp