The maximum image width in the blogging space of the Bueno theme is 490pixels. In Bueno the sidebar width is 300 pixels and the footer width is 280pixels. Since there is no official Staff support for the CSS upgrade, and precious little Volunteer support, I recommend that you don’t buy the upgrade unless you have at least moderate understanding of HTML and CSS because you must be prepared to do the work on your own.
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 have CSS editing experience, you can get the paid custom CSS upgrade and renew it annually. It will allow you to stylize the appearance of themes you find here > Appearance > Themes, but it does not allow you to change the functionality, by editing the underlying template. Also note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM.
If you do deiced to purchase the CSS upgrade then please be patient and consult tutorials found here > http://en.support.wordpress.com/custom-css/#css-help and this post http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/ while waiting for a Volunteer to help you. :)