500 Error after uploading new logo

Hello, I am new to Zencart.
After looking around here for awhile and not finding a suitable answer. I have decided to write the forums for expert advice.
My Problem is that I changed the logo on the main page - I uploaded a new image with same the filename (logo.gif)as the old one, no probs yet. I then went into the header.php file and changed the size of the logo from 192x64 to 400x100 - still no probs.
I then uploaded a higher quality image of the logo (mlogo.jpg), changed the filename in the header.php file and now I get a 500 Server Error.:oops:
I have since returned everything back the way it was, and deleted the new files, and still get the 500 Error.

Any Ideas?

Thanks in advance.

ps - I am using zencart v1.3.9 with cold steel template and godaddy hosting (yes I know goddaddy sucks, but its free right now)

I figured it out. Doh!!
it was just a missing apostrophe.
Amazing how 1 little syntax error can cause a site to crash!!

Thanks to Clyde for providing a link to the Debug Utility, which I never knew about till reading the Cold Steel Template Thread on page 52 or something like that.

ps - in the header.php file in includes/language/english/header.php - the filenames are CASE SENSITIVE!! Haven't noticed if the others php files are case sensitive, but when i do, I let ya's know