Using default template

Hi all. I’m very new to all this. Finally got WordPress all installed and everything, and it looks extremely easy and intuitive. Here’s my question: My WordPress blog is going to be my main page. How do I change the nice gradient at the top (Default Template) to an image? I have a cool logo I’d like to you. Here’s my other question: How do I put Google AdSense ads in the sidebar? Thanks for your help.