kouliga You will require to change this image and upload image with solely the top bg icon (Blue). Thereon, You can upload the image to similar directory path
Further, You can upload the image (white) you wish to show on hover effect and try add this to css file :
.department-list ul.department-list li.icon-4 a:hover{background: url("../images/ico/dep-4.png");}
and change bg image path in accordance with the image file name
Also, I would suggest to go through some of the relative examples here and here where you can test and try/learn as to how to change Background Image on Hover CSS