We were able to fix this issue.
Please add the image to the cafProject/webcontent folder and add the below css style:
background-image:url(#{facesContext.externalContext.requestContextPath}/Images/bckgroundImage.png);
This fixed the issue. Just wanted to pass this on so that this will help people who are looking to implement same feature in caf.
#webMethods#webMethods-BPMS#MWS-CAF-Task-Engine