Well, assuming you're trying to do it at the site in your signature, you'll need access to your site's CSS. I don't have much experience with the newer versions of IPB, but i'm pretty sure you need to find this:
PHP:
#ipbwrapper
{
margin: 20px auto 20px auto;
text-align: left;
width: 90%;
As for the blocks, i'm sorry, i really don't know. I remember how to do it in 1.3 final, but i have a feeling it will have changed since. They're called "pip" images, if that helps at all.