Here's the step to follow:
- Prepare a innocent JPEG file and a list of confidential documents (pdf, doc, ppt, xls).
- Compress the list of confidential documents using any archiving tool, 7-zip, WinRAR, WinZIP, said confidential.7z (this works the same for RAR/ZIP/etc).
- Optional, you can remove the extension of the archive file if you want.
- Now issue the command to join the JPEG file and the archive file.
copy /B innocent.jpg+confidential new-innocent.jpg
The new-innocent.jpg file will embed the confidential documents and is ready to be distributed. To retrieve the documents back, simply open the new-innocent.jpg file with any of the archiver tool and extract the files.
You will notice the size of the new-innocent.jpg is the total of the innocent.jpg and the confidential.7z.
If you open the new-innocent.jpg with any graphic editor, try to save it once, all your "attachments" will lost.