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It appears that images in tooltip-mode tooltips is currently broken on
most platforms besides windows. The following snippet should provide an
example:

(defun my-download-as-string (url)
  "Download file from URL and return it as string."
  (let ((image-buf (url-retrieve-synchronously url)))
    (when image-buf
      (with-current-buffer image-buf
        (goto-char (point-min))
        (when (looking-at "HTTP/")
          (delete-region (point-min)
                         (progn (re-search-forward "\n[\n]+")
                                (point))))
        (buffer-substring-no-properties (point-min) (point-max))))))

(defun my-download-image (url)
  "Download URL as image."
  (create-image (my-download-as-string url) (image-type-from-file-name url)
t))

(with-current-buffer (get-buffer-create "*TooltipTest*")
  (tooltip-mode)
  (insert
   (propertize "Facepalm" 'help-echo
               (propertize " " 'display
                           (my-download-image "
https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Paris_Tuileries_Garden_Facepalm_statue.jpg/300px-Paris_Tuileries_Garden_Facepalm_statue.jpg
"))))
  (display-buffer (current-buffer)))

Just `eval-buffer` then hover your mouse over `Facepalm` in the new
buffer.

Emacs versions 26.2 26.3 and 28.0.50 were tested on NixOS, Ubuntu, OSX
and Windows.

On Windows a tooltip displays an image. On Linux (NixOS, Ubuntu) and OSX
only a small empty tooltip is presented.

One package I was able to identify that uses this behavior is Dired+

This feature is also useful in general to maintain the readability of
org-mode documents for example.