[Roll Over Image] Quick Question !!! Urgent Plz!!!

Hello, everyone:

Wonder when set up a css rollover image, how to set the background-image correctly?
For example, if the image url is :
https://testsite.com/trail1/wp-content/uploads/2011/12/Tabs-trial-v_03.png
then, what should I write down in the CSS for my rollover’s background:
background-image: url(yaprak_bw_color.png);

Urgent help needed plz!!!!!!!