To change the theme you have to go into your FTP webserver.
go into themes/zoneCopper/images and find logo.gif
Delete logo.gif, upload that banner you made and rename it as logo.gif
Then go on your site and check it out, if it is too small then you have to edit the page file, normally header.htm or overall_header.htm
Download either header.htm or overall_header.htm and open it with notepad.
Then in notepad find the line where it says .../themes/zoneCopper/images/logo.jpg - Width= "700" Height="423" or something that looks like that.
All you have to do is edit the width and height sizes to the same size as the banner you made.
To find out the width and height of the ur image, just use photoshop.
Open the file, copy it,(making sure there is only one layer ofcourse) and then make a new file and select Clipboard, then just read the details it has, in photoshop it detects the clipboard's data height and width, just copy the numbers for each and paste them into the notepad file, then save and upload to your webserver, overwriting the old header.htm or overall_header.htm file.