aaargh - blogger header picture

i just noticed that i now can easily add a header picture but it's soooo frustrating, i don't seem to be able to get it in at the right size or ratio! i think i spent an hour half heartedly messing with it, and now i've given up... i'd be a lot happier if i could get it to be centred in the header border, but even that i can't seem to get to work... i managed to crop one picture so it's not aligned left any more, but it's still aligned to the bottom and doesn't look great. i'm not used to looking at code any more... help, anybody???