How to combine pngs?

[DAL59]

I saw this on the internet.  If you open this image against a dark background(click open image in new tab while in incognito mode), it shows a second image.  

 

[0111narwhalz]

It looks like alpha-channel shenanigans to me. One image (the foreground) is transparent, and the other is opaque, on a half-transparent field.

[Superfluous J]

I can see it even on the white background. So no real trickery just transparency.

[YNM]

At least I also see the outline here.

[kerbiloid]

Opera: works perfectly
In html only the foreground picture is visible.
If right-click and open in a new tab - the background picture.

Firefox:
In html only the foreground picture is visible.
If right-click and open in a new tab - the shaded background picture is visible behind the foreground picture.

Chrome.
In html only the foreground picture is visible.
If right-click and open in a new tab - the shaded foreground picture is visible behind the background picture.

XnView
Background picture only

TotalCommander, F3.
Transparent picture with clear foreground picture and shaded background one.

Gimp.
One-layer file with both pictures at once.

Paint.Net.
One-layer file with both pictures at once.
Transparent picture with clear foreground picture and shaded background one.

Inkscape (import)
The same.

MS Paint.
Foreground only.

[Mad Rocket Scientist]

Here you go:

While you're on that channel: https://www.youtube.com/watch?v=uNjxe8ShM-8

 

[roboslacker]

That image reminds me of SCP-513.

[DAL59]

found another one

https://i.redd.it/mcrwarpib7u11.png