.NET has .zip functionality as of FrameWork 4.5. : [
^].
If the copy /b technique is okay for you, then you could use that. But, of course, anyone with any smarts can force your file that appears to be .jpg or .png to open with WinRar or other archive readers.
You can invoke the DOS copy-merge command in .NET: here's an example: [
^].
There are lots of tutorials here on CodeProject on using zip, rar, 7-zip, in .NET: just search for them.
Here's a May 2012 tutorial on using FrameWork 4.5's zip facilities: [
^].