Hi,
The default image is located in your root xcart directory
So if your x cart directory is
http://www.yoursite.com/xcart,
the image is right in your first folder, in this case /xcart.
To change the image , rename the default_image.gif to default_image_old.gif and upload your new image named default_image.gif.Check if it is working and delete the default_image_old.gif.