[racket] Servlet static file paths, I just don't get it

This is driving me nuts, I just cannot find the place where my servlet
gets static files from. I run the following "server.scm" in a directory
in which there is also "htdocs/images/Sloth.png" but no matter what
combination of img src file paths and extra-file-paths I try,
respond-unknown does not want to display the Sloth.png when I click run:

#lang racket
(require web-server/web-server
         web-server/dispatch
          web-server/servlet-env
          web-server/servlet)
(require "lottery/darkstar-lottery.scm")

(define (respond-unknown req)
  (response/xexpr '(html
     (head (title "The 3-Toed Sloth"))
     (body
      (img ([src "images/Sloth.png"]))
      (h1 "Under construction")))))

(define-values (dispatch input-url)
  (dispatch-rules
   (("lottery") lottery-app)
   (else respond-unknown)))

(serve/servlet dispatch ; answers requests
               #:servlet-path "" ; is the default URL
               #:extra-files-paths (list (build-path
               (current-directory) "htdocs")) 
               #:port 8080 ; is the port
               #:servlet-regexp #rx"")

How do I make the servlet serve the file "htdocs/images/Sloth.png"
relative to the servlet source directory and how do I address it in the
"img" tag? I tried absolute paths, too, but no luck.

Best,

Erich

P.S. For completeness, the content of
lottery/darkstar-lottery.scm:

#lang racket
(require web-server/servlet
         web-server/servlet-env)
(provide lottery-app)

(define (draw n total)
  (sort
   (take
    (shuffle (for/list ((i (in-range total)))
               (add1 i)))
    n)
   <))

(define (stringify numbers)
  (string-append
   (apply string-append
          (for/list ([n (in-list (reverse (cdr (reverse numbers))))])
            (format "~a, " n)))
   (format "~a" (car (reverse numbers)))))
  
(define (euromillions)
  (define (draw-5-of-50)
    (draw 5 50))
  (define (draw-2-of-11)
    (draw 2 11))
  (format "The next lottery result is ~a with additional numbers ~a."
          (stringify (draw-5-of-50))
          (stringify (draw-2-of-11))))
    
(define (lottery-app req)
  (response/xexpr
   `(html (head (title "Euromillions"))
          (body (p ,(euromillions))))))
 
;(serve/servlet lottery-app
;               #:servlet-path "/lottery"
;               #:port 8080
;               #:command-line #t)