Why thumbnail shows just another pic? Download this picture and see it as a thumbnail, like in whatsapp or your phone gallery and you'll find that it shows another picture in the thumbnail.

If you can't see the thumbnail:
You can simply do some changes in color, or do some filters on it using PicsArt and you'll notice the difference.  I've searched the whole internet, but haven't got the answer yet.
Please explain how is this happening. 

Note: I know about the trick with PNG files, but this is a JPEG file and the trick works with this too.
I added video a to elaborate.
You can't see it in the image you uploaded because it's a JPEG, but PNG files which have a full alpha channel can do the trick you mentioned. It works with websites or apps where the "thumbnail" image is displayed on a white background and the "full" image is displayed on a black background. In the thumbnail display, white pixels and completely transparent pixels look the same, because of the white background; in the full image, black pixels and completely transparent pixels look the same.

Thus, you can have a white-on-black picture and a black-on-white picture in the same image, and the background hides one of the two images. You can easily see what's going on if you look at the image against a background of some other colour.