WordPress theme: Bluebell, Custom logo option not working

Ok, so within the theme options in the appearance menu on the left with the bluebell theme you have the option to use a custom logo instead of the default which of course is an image of a bluebell… every time I use the url of the image I want it comes up with an 404 error page. so something in the css or php isn’t right.. I know that much. So when I go into the theme editor and check out the theme functions php I find the part of the php script I want, change the logo image url in the call image script but nothing happens. I have no idea where I am going wrong and would really like some help or screenshots of where it needs to be etc… thanks in advance. im out and about until around seven o’clock so don’t be surprised if im silent until then.