A little over a year ago I wrote about a plugin that with set a default image for post thumbnails if one wasn’t set.
Post thumbnails were added to WordPress in version 2.9 so if your site was older than that, your older posts wouldn’t have one set (since they didn’t exist). And if you have a longer attention span than I do, you could always go back through your older posts and set a post thumbnail in the featured image section. Of course, the code for post thumbnails does need to be added to your template files first.
