What was this method again? Something to do with transparent PNG

laisy said:

What was this method again? Something to do with transparent PNG

It's a white background where the darker parts are semitransparent. I see her clearly on my dark Danbooru background but switching to another domain I'm not logged in to (thus a white background) makes her vanish. Using my checker bookmark which turns any transparency "pure" bright red:

javascript:document.getElementById("image").style.backgroundColor="red";void(0)

results in various intensities of pink.

I have no idea how actual PNG code works so I dunno if there's a "setting" for semitransparency or if it's just one transparent pixel per X dark pixels.