Header? Hello!

Okay, at risk of sounding completely daft, I must say that I have been here for hours trying to fashion the darn matching header for my blog but for some reason when I upload it to the header portion it shrinks and only fills up the left half of the header area! What gives? Is it a Blogger issue or what? I followed the directions to a tee and tried it several times (more than I would like to mention). Please help. If anyone knows how to do this please share! With that said, I am going to bed...