image url in CSS background-image

hi

 I want use calss with this code in css

.maindiv
{
    background-image:url('image/8.jpg');
    background-repeat:no-repeat;
    height:2600px;
    width:1024px;
    }

when i use this class for my div it didn't show image but when i change url address like this

.maindiv
{
    background-image:url('http://localhost:3723/mysite/image/8.jpg');
    background-repeat:no-repeat;
    height:2600px;
    width:1024px;
    }

it showed my image why?

in past always i use url like this

    background-image:url('image/8.jpg');

and that showed my image but i don't know why  it didn't show my image now?