Changing Logo

I am new to wordpress and I am having trouble changing my logo. I purchased the bigfoot theme from theme junkie a couple days ago. At the top on the header is a picture of the bigfoot logo. I have been going into appearance and changing it under bigfoot theme options. When I enter the bigfoot theme options, I have been entering the url location of the png image I created. This has been working, but the second I sign out the image goes back to the bigfoot theme logo. I was wondering if anyone has encountered similar problems or knows how to fix this. I would like to avoid code if possible.

Thanks,
Matt