I think that just by passing a 640x480 16bit bitmap in bitmap= argument you achieve what you're trying to do. (Comment the line that says message= since this two lines can't be togheter).
I've just found this directory on my system: /usr/doc/lilo-22.5.9
There you can read README and README.bitmaps and also see inside the sample subdirectory where you've some sample bitmaps
Update: I tried this feature, I've a graphic lilo selection now

I just simply edited /etc/lilo.conf, i commented the line #message= put a line with bitmap=/usr/doc/lilo-22.5.9/sample/ins64a.bmp
Then I run: lilo -v -v
All seemed fine, rebooted and it was there
