Jess

Thumbnail Updater does exactly what it’s name implies: it updates thumbnails. WordPress 2.9 introduced support for 
post thumbnails but one problem is that the support is not backwards compatible—meaning that if you downloaded or created 
a theme that supports post thumbnails, any images uploaded prior to the activation of that theme will not have the new thumbnail size.

Thumbnail Updater

A plugin for updating your thumbnails whenever a new thumbnail size is added with add_image_size()

Travis Smith

Genesis Featured Images creates a option box to enable a default image for post thumbnails and/or featured images and a meta box to set a custom size for the featured image to be displayed. These default images will only appear any time a post, page or any custom post type does not have a featured image, or an image inside the post. So for those who write a lot of posts and want one image for the majority of your posts, this plugin works perfectly! Genesis Featured Images filters genesis_get_image() so anywhere you utilize genesis_get_image(), this plugin will provide a fallback option. Genesis Featured Images uses the WordPress Image Uploader and Library to upload the image to ensure that the proper thumbnails are created.
IMPORTANT: 
You must have <a href="http://wpsmith.net/get-genesis" title="Learn more about Genesis" rel="nofollow ugc">Genesis</a> installed. Click <a href="http://wpsmith.net/get-genesis" title="Learn more about Genesis" rel="nofollow ugc">here</a> to learn more about <a href="http://wpsmith.net/get-genesis" title="Learn more about Genesis" rel="nofollow ugc">Genesis</a>
<h3>Special Thanks</h3>
I owe a huge debt of gratitude to all the folks at <a href="http://wpsmith.net/go/studiopress" title="StudioPress" rel="nofollow ugc">StudioPress</a>, their <a href="http://wpsmith.net/go/spthemes" title="StudioPress Themes" rel="nofollow ugc">themes</a> make life easier.
And thanks to the various individuals who helped me through the beta testing.

Genesis Featured Images

This plugin sets a default image for post thumbnails for the Genesis framework.

Thumbnail Updater vs Genesis Featured Images