First, disable the beastie menu:

in /boot/loader.conf:

beastie_disable="YES"
autoboot_delay="-1"

Then you can choose between a silent boot or a splash screen(bmp):

-- Silent boot:

echo "-nmq" > /boot.config, as root.

-- Splash screen:

in /boot/loader.conf, the image must be a bmp and can not be more than 1024x768.

vesa_load="YES"
bitmap_load="YES"
splash_bmp_load="YES"
bitmap_name="/boot/splash.bmp"


-- Silent rc output:

in rc.conf:

rc_startmsgs="NO"