[fasterthanlight]

It is not a kind of magic. It is only possible if the picture contains exif data with thumbnail. Check it with irfan by pressing i and e.
The most new digital cameras join the picture with many other informations like camera setups,date... AND a thumbnail picture.
Remark: "Canon store the owner name, too - sometimes very interesting ;-)".

For example W-XP explorer use this little picture to make the preview in a folder. If you see in the explorer preview the black box then is no way to recover this picture. The thumbnail was updated.

You need a program to extract the little picture from the original picture. I use the freeware exifer.
There is a preview windows and with the right mouse button you can store it as separate file.

Now you have 2 jpg-files- the original and the thumbnail picture.

The next steps join this pictures:

1.)start gimp and open the original picture

2.)move with drag and drop the thumbnail-file into the layer window, one the right site. Now you have create a second layer with this picture. It is important that the original layer is above the thumbnail picture
L1-Original picture
L2-Resized thumbnail

3.) select this layer and rescale this picture, best way to use the values of the original picture. Booth pictures(layers) must have the same size.

4.) take the magic stick and select the black box area on the other original layer. Now you have a selection

5.) swap the black color with transparent color.
color-> color to transparent. Now you see the other layer through the original picture.

Save - that's all

good luck