Also, when blacking out data from JPEGs, leave generous margins, because there may be compression artifacts in the nearby pixels that also act effectively as a one-way hash. I haven't tested how consistent these artifacts are, but it's better to be safe than sorry.

Do they? First I’ve heard of that…

The thumbnail is part of the EXIF metadata. It's reasonably well-known now, but it was not always the case... http://graphicssoft.about.com/b/2003/07/26/techtvs-cat-schwa...

Huh, interesting. Thanks!

A few years ago a TV presenter got caught out by this, she posted a cropped photo of herself online, but the original, uncropped version could be recovered from the file which showed her topless.

There was a trend for a while of people posting images to imgur that had a totally different thumbnail. A rickroll of sorts.

I believe it's a Photoshop thing, rather than something common to all images. Photoshop in general does unusual things.

See also: http://u88.n24.queensu.ca/exiftool/forum/index.php?topic=451...

You can use 'exiftool' on Linux to manipulate the EXIF data, get rid of the thumbnail, or set it to whatever you want. Imagemagick's 'convert' command also has a '-strip' option that will get rid of all EXIF data including any thumbnails.