Thumbnail support

Hi,
I have the scarlett theme as my theme and there is a slideshow like at the header and a small one on the right hand side. I couldn’t find how to get the images of posts up there. I installed the thumbnail for experts plugin. Basically the plugin says it takes the first image it can find on the post and uses that as the image. I have activated it and uploaded an image onto a post and it still isn’t working!!! Anyone know why?

Thanks a mill