some 2htdp/image equality clarification

pkgs/htdp/teachpack/2htdp/scribblings/image-guide.scrbl

@@ -479,7 +479,7 @@ color in it:
                     (rectangle 100 100 'solid (make-color 1 1 1 50))
                     (rectangle 100 100 'solid (make-color 2 2 2 50)))]
 To understand why, we must look more carefully at how alpha blending
-and image equality work. Image equality is straightforward: two images
+and image equality work. Image equality's definition is straightforward: two images
 are equality if they are both drawn the same. That is, image equality
 is defined by simply drawing the two shapes on a white background and
 then comparing all of the pixels for the two drawings

pkgs/htdp/teachpack/2htdp/scribblings/image.scrbl

@@ -1600,6 +1600,12 @@ Two images are @racket[equal?] if they draw exactly the same way at their curren
 (not necessarily at all sizes) and, if there are pinholes, the pinholes are
 in the same place.
 
+This can lead to some counter-intuitive results. For example, two completely shapes that
+are the same size and are drawn with the transparent color are equal:
+@image-examples[(equal? (circle 30 "solid" "transparent")
+                        (square 60 "solid" "transparent"))]
+See also @secref["nitty-gritty-alpha"].
+
 @section[#:tag "pinholes"]{Pinholes}
 
 A pinhole is an optional property of an image that identifies a point somewhere