Well it’s no fun if I do it for ya ??
All WordPress themes have default files that are required, the one you’re looking for is header.php and it’s located in the themes folder:
%wordpress%/wp-content/themes/twenty-ten/header.php
%wordpress% either being your domain name, or your wordpress folder, which can be renamed to something different of course.
Now just open that file with your favorite text editor, notepad should suffice. Then do a ctrl+f (Find) and search for that block of text above. Once you find it, just replace it with your image of choice, keeping in mind the path relative to the WordPress root directory.
To add an image you’ll need basic HTML knowledge, you can see the “img” tag here.
Also you should note that if the image is in the following directory:
%wordpress%/images/
You’ll have no problems with using the img source as that. However, if you’re using a theme, you WILL have to use PHP in the img source.
Use bloginfo for template development
Use WP_PLUGIN_URL/%yourPluginFolder%/ for plugin development.