$content_width not doing anyhting

I’ve set overflow to visible in the main content area for facebook stuff and now all of my images and youtube embeds are overflowing by about 10 pixels:

http://costaricadownunder.com/?lang=en

the videos are fine now because I put a manual resize in the posts, but you can see the images farther down are not.

I could resize all the images, and from here on upload the images at the correct width, and put in little embed tags, but I would rather that $content_width do the job automatically……

here it is in function.php, right after the opening:

<?php

if ( ! isset( $content_width ) ) $content_width = 640;

automatically included with my theme, there must be something overwriting it? where could I look?

PS: theres another thing, when I use zoom with ctrl + mousewheel in and out the images gradually get bigger, is this a related problem?

double PS: ive noticed that if I click refresh and watch the images, the start off at the correct size and a few seconds later they expand.