How do I get a picture on the left or across the whole page?
Please read the post at the link airodyssey posted and note these images sizes:
Small: 440 x 240 on the home page for featured posts
Medium: 620 x 380 on the home page for latest feature post
Large: 640 x 392 on single posts
The showcased post is the latest post from a given category; category selected in Appearance > Theme Options > “Featured Category” dropdown.
If you want an image that spans the whole page then you will have to purchase a CSS editing upgrade and take on that challenge yourself as there is no Staff support for CSS editing and precious little Volunteer support.
If you are considering purchasing the upgrade, you can go to Appearance > Edit CSS. There wordpress has provided a preview function where you can try before you buy.
If you do purchase the upgrade be sure to read this article before starting your editing > http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/