To replace the stock image that comes with Cubecart 3, with your own personalised image, first download the header file. I use the Killer skin, if your using another theme/skin, replace Killer with the name of your theme/skin. Its located at:
You can edit the file or create a new file with the same or similar dimensions.
When your done, save the file and rename it to "topHeader.gif".
Upload the file to your server using FTP. Place it in the same directory it originally came from:
Your FTP client should ask if you want to replace the existing file. Click yes.
Go to your site and reload the page.