Customising the icons on overview page of ESS

Hi ,

I have a requirement for ESS in portal that i have to change the icons available in Overview page of ESS.

Say for example for near Wrking time we have a icon clock icon where i found this under the path spro->personal management->Define Resource-> and for the resource EMPLOYEE_WORKTIME_PIC i found the value for Object name: calendar_clock.gif.

Now i would like to know where this gif file is stored in portal so that i will replace my custom icon into that place.

Please let me know the path where this file is stored in the portal.

Thanks in advance.