Change a picture in a WordPress theme

I use a free Wordpress theme.? This is my first site ever.? I only would like to change the picture in the header.? I am a graphic designer so if you tell me the pixel size and the file format I can do it on my own. I just don't know hoe to insert it.? The rest of the design is kind of okay.? Actually the design doesn't matter that much because this is going to be a blog. A blog about small kitchens. So I want a header relevant to this.