Gallery issue – rectangle

Hi everyone,

I watched some tutorials to make a photo gallery on WordPress.org without using any plugin.
http://codex.wordpress.org/The_WordPress_Gallery

But, when I upload my photos and make the gallery, a grey rectangle appears around the pictures in the “preview mode”… i just want it in square like here:
http://en.support.wordpress.com/images/gallery/

Do you know how can I do?
Thank you for your help !