bug#57813: Icon images are non-functional

[Juri Linkov]

(info "(elisp) Icons") suggests to use:

     (define-icon outline-open button
       '((image "right.svg" "open.xpm" "open.pbm" :height line)
         (emoji "▶️")
         (symbol "▶" "➤")
         (text "open" :face icon-button))
       "Icon used for buttons for opening a section in outline buffers."
       :version "29.1"
       :help-echo "Open this section")

So I tried this with 'C-h b', but images are displayed as black boxes.

PS: "right.svg" exists in etc/images/right.svg.