Ok. FEW Words on it.
what I have done in one project for LOGO.
Upload image from se78.
You can copy standard form HR_ESS_PAYSLIP_TO_PDF to Z one ZHR_ESS_PAYSLIP_TO_PDF.
then make changes in window and insert logo on it like image
Then calling that one through z program like this
IF P_FORM[] IS NOT INITIAL.
P_FORM1 = P_FORM[].
CALL FUNCTION 'SSF_FUNCTION_MODULE_NAME'
EXPORTING
FORMNAME = 'ZHR_ESS_PAYSLIP_TO_PDF1'
IMPORTING
FM_NAME = FNAME
EXCEPTIONS
OTHERS = 3.
You can try like this.
Otherwise go for HRFORMS.
also look at these thread ,http://scn.sap.com/thread/2071894
http://scn.sap.com/thread/521050
It will also help you.
Thanks
Gourav.