Add a camera with the button in the panel or with shift A, and you select a border in the previews.
That border will be added to the front of the camera,
The Border type selector has a choice of 5 different borders that can be added.

If you select one of The look through types, there will be an image added to the camera lens.
Like you are looking through a binocular, a keyhole from a door, a sniper scope, a window, and so on.
The image can be switched any time you like.
You can have more than 1 camera that has a border or look through frame in front of the camera.

The selected image will always stick to the camera if the camera is rotated, moved or shifted.
If you want to use your own images, just set your path to your images folder and select your png image.
But make sure that your image is transparent.
The restore camera button will erase the selection from your camera and the camera will be a default camera.