It's password-protected. To use it, you simply double-click the image and enter your password. A virtual disk will mount on your desktop, allowing you to copy to and read from the image. When you're done, simply eject that mounted image by highlighting it and pressing Command-E or dragging it to the trash (make sure you're dragging the mounted image, not the disk image itself!)
Others can do the same, but without the password, it's useless to them.