Yes, it is, and very easy as well.
The page linked to above is good, if you want to use your own graphics.
There is an easier way, using some already made, Apple orientated startup screens. I can't remember the url now, but if you search in the OS X forum, it shouldnt be too hard to find, its called something like BootX (probably named after the file you replace in System folder.
If you cant find it, email me and I will send you the replacement files.
Hope this helps.
Tom.