Try the following starting from emacs -Q + the attached .png file:

1.

(let ((ov (make-overlay 1 10)))
  (set-frame-parameter nil 'alpha-background 90)
  (overlay-put
   ov
   'display
   (create-image
    "/path/to/image.png"
    nil nil :width 100)))

2. Observe image background not being transparent, but instead using
default face background.

3.

(let ((ov (make-overlay 1 10)))
  (set-frame-parameter nil 'alpha-background 90)
  (overlay-put
   ov
   'display
   (create-image
    "/path/to/image.png"
    nil nil :width 100 :mask 'heuristic)))

4. With :mask 'heuristic, the background becomes transparent (mod
"rough" edges due to clipping)

I believe that (1) is a bug.

See also https://list.orgmode.org/orgmode/7324dc0.f8517.195a224ffd1.Coremail.dybfysiat@163.com/