Here is an image:

But it is also an HTML page. Here is the markup for the above image:

<img src="http://lcamtuf.coredump.cx/squirrel/" />

Now try visiting that webpage:

http://lcamtuf.coredump.cx/squirrel/

Pretty cool, eh?

I did not make this. I am just showing you because it is cool.

Here is an explanation of what is happening:

The file is a valid jpeg file with some html in metadata. The server responds with Content-Type: text/html, making browser interpret the response body as html. Browser ignores jpeg header until it sees and starts to render the html. The html in the jpeg metadata ends with "<!--" , which starts html comment. The html has that renders the file as image.

I found this hack fascinating, and I hope you did too.