Verified answer

Photoshop-shop-shop.

If you know anything about select>copy>paste, it's easy to figure out. If you don't, it would take forever to explain it here.

Take a shot of someone holding a picture.

Select the whole picture.

Copy it and drop it into a new layer.

Shrink it to the same size as the picture your subject is holding.

Repeat, Repeat, Repeat.

Droste effect. It's basically a Copy-Paste-Resize procedure done over and over again. You can do it using a capable graphics program like Photoshop, Corel or Gimp.

http://keerok-potd.blogspot.com/2012/10/within.htm...