An internet prank that makes you think someone hacked your computer and took a screenshot of your hard disk content. Although the file has a .jpg extension, the actual content is HTML. This is the content of it, stripped down to the basics:
<html>
<head>
<title>hacked.jpg</title>
</head>
<body>
<iframe src="C:\" width="800" height="600">
</iframe>
</body>
</html>
The trick in this is the use of the <iframe> tag, which displays the root of your C:\. This happens locally, there is no way the so said hacker can actually see what's on your computer. When you (double/right) click on one of the folders or files displayed, you'll notice it doesn't behave like a .jpg at all.
As far as I know it works on any Windows system with a recent Internet Explorer.
Opera browsers aren't fooled by this joke because Opera doesn't recognize C:\ as a supported address type. It does recognize file://c:/ (which is for some reason rarely found in a hacked.jpg, although it also works in IE), but displays it in a way that has little resemblance to the appearance of Windows Explorer.
Mozilla Firefox doesn't fall for this trick at all. It tries and reads the file as a JPEG image, and when that fails it'll simply display the URL of the file. It does not interpret the HTML code, although it does show it when you view the source.