The only trick is you need WinRAR (which I use anyway, daily).

Rar up your TXT file and put it in the same directory as your JPG file. Then open a commandline window, nav to the directory, and type:

copy /b originalimage.jpg + secrettextfile.rar destination.jpg

(obviously you'd put in your own filenames)

Attached is an example I made (thanks to Chris H for the image). You can view it as normal, but it has a little secret message inside if you open it with WinRAR.