I've been asked, how to distribute a secure file that can be camoflaged to look benign as a picture and pass through a firewall, using port 80 (HTTP). Simple.
You need WinRAR or other file compression tool, but RAR is quite simple to use.
Collect your files that you want to hide and compress into one by using Winrar.
Suppose 'data_files.rar' as the filename
Select an image file (picture.jpg) for the destination, 'image.jpg'
Put the two files (File picture.jpg and data_files.rar) in a same folder.
For example, c:\tempDrive
Open a command prompt as admin, in that directory and type:
c:\tempdrive\> copy / b + picture.jpg data_files.rar newpicture.jpg
That will create a new file with a name newpicture.jpg.
When you open the file newpicture.jpg, at first glance there would be no different than your original picture.jpg file unless you view the file size, now larger. You can still open the file with Picture Editor or Irfan and it display as normal. There are no signs that there are actually an additional file in it.
How to unlock your secret files?
Right-click on the file picture.jpg then choose [Open With] -> [Choose Program ..]. Select WinRar and then click [OK].
You will be able to view your both the .jpg and .rar files and can extract as usual.