If you've got the new header image for your Wordpress theme already, you'd be looking at around 10 to 15 minutes work, with testing, at most (and that's being generous with time!)

Not my thing but if you get stuck you can try the official IRC Wordpress chat channel on freenode and ask there. There is also an email list you can request help on called wp-pro. Google up either of them for more information.

I´m a graphic/web designer and developer with tons of experience in wordpress, i´ll be glad to create a custom header for you and apply it to your blog. Please contact me if you´re interested. Thanks

Assuming that you have created an image that is the same size you could just name it identically to your existing header image (assuming that it's the same image format) and just overwrite it. Should only take a few minutes.

If it isn't the same format but is the same size you'll have to edit the name in either your header.php or css style sheet.

Bill