Use the same thumbnail image on multiple posts

I’m setting up a new installation of WordPress as a CMS Intranet at my place of work. I’m using Theme Junkie’s free Channel theme as a base for building the site, but am having problems with the thumbnail images. Everything else on the site is working fine, all of the mod re-writes, images in the posts, search, etc. – but I can’t re-use media library images and have them display as thumbnails.

They’re displaying fine on individual posts, but if I add a new post and select an image from the media library (as opposed to uploading a new image), then it displays fine in the post, but no thumbnail shows on the home page summary of posts.

Can anyone help please? I’m hoping it must be a setting somewhere, as opposed to be needing to edit any code, but any help will be gratefully received – as I don’t want to have to re-upload the same “News” or “How to” images, each time I add new content to the site.

Thanks in advance,

Matt