You can try doing the following:
background-image: url(http://www.example.com/images/bck.png); // absolute path
background-image: url(rio.jpg); // relative path
If you are stating that the path is just rio.jpg, the image should be in the same directory as the stylesheet/where you are declaring the background.
If the image is in an image directory, you may need to go up a level as follows:
background-image: url(../images/rio.jpg);
Depends where the image file is located. You can refer to following for further information: https://developer.mozilla.org/en-US/docs/Web/CSS/background-image
Hope this helps!