Hi there,
Do you mean you want your post's images to appear next to your site's title in the header? If so then unfortunately that is not the way this theme's layout is designed.
http://theme.wordpress.com/themes/sight/
It's possible you might be able to display your featured posts in this way by manipulating the CSS through the Custom Design Upgrade, but it would require fairly advanced CSS skills.
http://en.support.wordpress.com/custom-design/custom-css/
Please let me know if I you have any other questions I can help you with.
Best!