excerpt with thumbnail goes to full post with large sized image

i don’t know why this is so difficult

i’m using the wordpress 2012 theme that has only been altered visually

i’d like to have my post page have excerpts with thumbnails and then the full post page is more customized with a full image and the thumbnail is not gone

something like this:
http://bit.ly/19rYFIY

here is an example someone else did that represents the same functionality i’m looking for:
http://newwp.templatepanic.com/?themedemo=Scratch

i have tried a million plugins, nothing seems to work quite the way i’d like it

i know you can use the ‘featured image’ to make thumbnails (which is confusing to me) but then the thumbnail persists on the single.php post page and i can’t seem to get it to shut off

there’s got a be an easier way? i’ve searched for a ridiculously long time and have had no luck.

thanks for any pointers you can throw my way.