size of image on theme twenty ten

I have just stated to make a website for a friend of mine.

I have removed the sidebare for the first page so that the image of the page will be as big as I can get. BUT – I cant seem to get it bigger.

You can see what I mean on this youtube clip that I made.

when I click the picture – it seem to open up in a different page and there it is bigger. but can I not make it work here?

Can it be solve by using css? and how?