initImage supports only jpeg images.

[Mustlane]

Reproducible:

JPEG:

s0.initImage("https://upload.wikimedia.org/wikipedia/commons/2/25/Hydra-Foto.jpg")
src(s0).out()

PNG:

s0.initImage("https://pngimg.com/uploads/penguin/pinguin_PNG15.png")
src(s0).out()

[Mustlane]

If it's an expected behaviour, then I think it would be nice if it was documented somewhere.