Thanks for your reply, FnCool,
I found a new way to upload my logo yesterday and I obtained this information from
http://support.bryght.com/drupal5/uploading-logo.
The above website showed me that I can actually upload my logo in administer, themes, uncheck "use the default logo", browse and upload the logo.
However, I tried to do exactly what it told me, but I still failed to upload the logo. No picture was displayed but only a cross. I made the logo transparent in Microsoft Word before uploading it. Am I doing the right thing? And it mentioned that I should resize to fit in a 183 by 82 pixel area. What it means by that? Should I resize the logo into 183 (weight) x 82 (height)?Is that what it means? I even tried to make a smaller size logo compare to Drupal logo, but I still encountered failure.
I am a newbie, hope you don't mind showing me the way. Thanks again.