What you are refering to as a Start Menu Picture is actually the User Account Picture. On a normal install they are located in C:\Documents and Settings\All Users\Application Data\Microsoft\User Account Pictures\Default Pictures. They are in the BMP format, 128x128 pixels, 49,208 bytes, 24-bit color
you need to edit your oobe.xml and place the path to your user picture usertilepath eg: here is one for hp : <usertilepath>C:\ProgramData\Hewlett-Packard\HP\user.bmp</usertilepath>