Homepage feature image / Fixed Width Page

Hi. I am a WP newbie and have installed the Pinboard theme.

I created a new page and made it the home page for my site and I want a large feature image like the homepage of the Pinboard theme but all I get is a WYSIWYG editor that creates content within the regular blog post box.

How do I insert the big image and make it use up the whole width of the theme?

I realise that is probably more of a general WP question. Also though, I would like to fix the width of the page, so that it doesn’t resize for different browser widths. How can I do that?

Many thanks
Bigus