It's not fading to "black transparent" or "white transparent". It's just showing whatever color is "behind" the image, which is not the image's background color - that color is completely hidden by the image.
If you want to fade to black(ish), you'll need a black container around the image. Something like:
.ctr {
margin: 0;
padding: 0;
background-color: black;
display: inline-block;
}
and
<div class="ctr"><img ... /></div>