Images loading fine on day 1, broken on day 2
There is a related issue for this here. http://drupal.org/node/1352182. 

What's happening is that drupal is storing the URL that is created when the page is first viewed after the cache is cleared. Normally that would be your site url, but sometimes it's not. 

In the issue linked you see that some people fixed this using the Pathologic module.