Thanks for the tip, Sebastian *hug*

What you’re doing is basically taking the contents from both files, the jpeg and the zip and dumping them into one big file.

JPEGs are read from the beginning of the file, and they are terminated with an “End of Image” marker. This means you can append anything to the end of a valid JPEG and it’ll still be a perfectly valid JPEG and work in any image editor.

ZIPs, on the other hand, have their metadata stored at the end of the file as relative positions. What this means is, you can append a ZIP file to any other file and it’ll still be a perfectly valid ZIP file.

Thus if you append a ZIP file to a JPEG file, it’ll be both a valid JPEG and a valid ZIP file.

To test that out, try following the guide and storing something inside an image. You can either name it secret.jpg and see it as a picture with the data hidden, or name it secret.zip and see it as an archive with the picture hidden.

Doing it on linux is no challenge, you simply do a:
cat image.jpg data.zip > secretimage.jpg
(“cat” is the *nix equivalent of the Windows “type” command.)

(I used to mess around a bit with this a while back. I believe it works with GIF, too.)

Sebastian Paaske Tørholm’s last blog post is A few things you should know about your favorite video sites.