different widths for image and text?

Hello, I was wondering if it’s possible to have different widths for text and image? I have seen it around in a few blogs, but I’m not that experienced with coding to figure it out. I would like large images at about 900px and the text to be at 400px, all centered.

I tried:

.post-body {
font-size: 110%;
line-height: 1.4;
width: 400px;
margin: 0 auto;
}
.post-body img {
padding: 2px;
background: #ffffff;
width: 900px;
display: inline;
float: left;
margin: 0 auto;
}

But with this the image only starts where the text starts, and is not centered. Help?

Thank you! 🙂