Open the following defination and changed them.
1. NEW_PS_LOGO (Image Type) - To change the logo.
2. PORTAL_UNI_HEADER_NS4X (Html Type) - Change logo name on mouse over.
3. PORTAL_UNI_HEARDER_NNS (Html Type)
4. PT_BRANDING (Application Package Type) - Deals to manipulate homepage and portal look.
Note: Dont change image name in PT_BRANDING application package. Have a backup of original Image type as NEW_PS_LOGO1 and update NEW_PS_LOGO with your image.