If you use the regular Site Settings > Change portal site properties and SharePoint site creation settings to insert a large banner image, you may find this expands the page too much as the Site Settings, Help and My Site links will get pushed further to the right. To get round this, create a transparent CSS file and insert this as the logo image, such as*:

/_layouts/1033/company_name/Images/transparent.gif

* This is because if you leave it blank, it defaults back to the standard SharePoint logo.

You'll note in other posts I talk about setting up a new directory called company_name. Under this folder I store other files such as the custom CSS file, images and scripts.

Then modify the CSS (in the custom CSS file you specified - see "Customizing the Portal") to insert the banner image as a background image.