Not sure you are still looking for a solution but on the Chipsee supplied "CD" you will find the following directory/folder:
\SoftWare\For BeagleBone Black\Linux-EZSDK\Logo modify\
In that directory/folder you find a Windows application (ChipSee_LOGO_MOD_EN.exe) that is specifically designed to easily change the BMP in the u-Boot.img which has the logo/splashscreen.
Good luck!
Dean