There are some fantastic docs discussing exactly what you are interested in. The menu at the top of the page takes you there (“Docs” in the menu) and there is much to read! If you would like to skip to the ‘customize the header’ section here is the link
http://codex.wordpress.org/Designing_Headers
I would highly suggest reading the introductions as well. If you don’t have much (or any) HTML or CSS experience this could be a challenge. However, there are programs out there that can assist as well as countless online tutorials. Google is your friend in this case.
If you’re not feeling up to the challenge of editing the theme yourself you can find some wonderful themes online. It’s as simple as uploading them and selecting them in your Admin panel of wordpress.
If all you want is a different header image, here is how as most of my blogs have a custom header image.
I look at the theme I use.
I find out what and where the jpg, gif, etc. it uses for a header image is located in the theme files.
I create or find the image I want to replace it with.
I use Irfanview to change the width and height of the new image to match the old image.
I save as and name the new image to the same name as used by the old image.
So if it is named header05.jpg then that is what I name the new one.
ftp the new header image to the location of the old one on your blog. I use ws_ftp, not the ‘upload’ part of the blog as that would put it in the wrong folder.
as an example:
I use the jewelry box theme for my dice blog. I substituted a dice image for the jewelery image and uploaded it.
