Facebook just updated their Like button to include an image by default for all "Likes". If the image isn't specifically specified, Facebook will look at the page and try to determine the 'best' image to use for the Like. We're working on a method for the 3.2 release for an admin to specify the image (and some other attributes) per page, and where possible, we'll try to automatically add the appropriate info.
If you'd like to do it manually, for now, even if it's hard-coded to the same image across your site, you can add the following tag to your template header sections:
<meta property="og:image" content="YOUR DEFAULT IMAGE"/>
For more information about the available tags, see:
developers.facebook.com/docs/opengraph/