Do you have the case correct? (e.g. is the file mycompany_logo.JPG ?) Windows doesn't care about the case but linux web servers do.
Is there something in your .htaccess file blocking it?
Type in your browser address bar
http://yourstore.com/path-to-xcart/skin1/images/mycompany_logo.jpg to see what you get.
Check your web server error logs to see whats happening.