Hello. Maybe I've found a bug.
This is what happens.
My image already has background transparency and it's already stored in Photos.
If I add it to a Pages document it retains transparency.
If I edit the very same image with pixelmator I can see that the background is transparent.
If I save the image from Pixelmator to Photos, then I add it to Pages document, the transparency is gone.
If I open the new image with Pixelmator the transparency is gone as well.