Web Design Help with editing a header image in Nvu

I am so confused my head is spinning. Somebody might have already answered my question but I've looked over soooo many of other people's questions already that I think I'm even more confused now.

I am using Nvu to edit my template for my website. I can also use SeaMonkey if that helps anybody help me. I can edit all of the text and even insert pictures in place of the generic pics on the template. However, I cannot figure out how to change the one big picture at the top. I am using the dog care template. When I look at the html source it does not show anything about that picture. All of the other pictures have a path so that I can change it with my own picture but this one has apparently nothing. When I look at the files I downloaded with the template the picture is titled header.jpg in the images folder. That is all I know.

I am using a Mac so I don't know if that makes a difference. If you need any more info or links or anything let me know I'll be happy to provide them. I'm a complete beginner at working with templates. I've only had experience with Frontpage and now that I'm on a Mac it's a whole different ball game.