How a face ends up inside a costume
Each costume is a studio photograph of a plush outfit with the face left empty. Your photo goes underneath it and the costume sits on top, so the face shows through the opening. The opening was cut once, carefully, before the file ever reached this page: its edge follows the individual strands of fur, which is what stops the join from looking like a sticker. Your browser does no colour work at all — it draws two pictures, which is why the preview reacts the moment you drag.
Which photos work best
A photo taken straight on, in even light, with the whole face visible. The costumes were all photographed facing the camera, so a face turned to the side will not sit naturally in the opening no matter how you drag it. Strong shadows across the face have the same problem: the costume is lit softly and evenly, and a hard shadow gives the join away. A plain phone photo taken indoors near a window is usually ideal.
The faded picture around the hood
While you are framing, the rest of your photo is drawn over the costume at low opacity. Without it you would be aiming blind: only the slice of face inside the opening would be visible, and when the chin or the forehead disappears there is no way to tell which direction fixes it. The faded layer is drawn from your photo and the costume's own transparency, so it stops exactly where the opening begins. It exists only on screen. The file you download contains only what shows through the hole.
What this tool does not do
It does not find the face for you, it does not rotate or straighten anything, and it does not change the background of your photo. Those are all places where an automatic guess goes wrong silently, and a wrong guess on a photo of your own child is worse than a slider. The framing is yours, and you can see the result before you commit to it.
Why it runs in your browser
A photo of a child is about the most personal file a person has. Uploading one to a free website means handing it to a service that has no reason to keep it and no obligation to tell you what it did with it. Putting one picture on top of another is simple arithmetic on a canvas; your own machine does it instantly, and the photo never leaves the device it is on.