Review the correct syntax for background image as suggested. It will look like this example:
background: url('../images/pano5_gradient_bar1.jpg') top center no-repeat;
Also realize that if the background image is being called from an external style sheet, the path to the image is relative to (starting at) the .css file, not the Web page it's being used on.
If the css is directly on the Web page... then you can use a path relative to the Web page.
So in the example I posted above, the "style_sheets" folder is at the same level as my images folder.... so to use that background image (starting at the css file), I have to go up one folder level (the ../ part of the path), then into the images folder.
In your example:
You are saying that the images folder is one level below the .css file (which may or may not be correct.
And yes, you do need to use the colon: not the single period... as mentioned.