Header anyone?

I've been sitting here for about an hour and a half trying to figure out how to edit my blog header and insert my own pictures, but every time I load a picture on, it squishes it down and it looks ridiculous! I've seen Kendall's and Rachel's and theirs look awesome, but I'm at a standstill. I've worked with html, picasa, googlepages (which I may actually like better than blogspot) so we'll see how things work. Dang headers!