Noobie needs help with mag style layout

I’m having some trouble putting together a magazine style layout (teaser on the front page with a thumbnail, full article after clicking the link).

I have a plugin installed to automatically create a thumbnail image out of the first image in a post. However, I have the following issue.

How do I prevent the big full-size image from appearing on the front page? Right now I just get the thumbnail right next to the full size image on the main page.