The thumbnail feature has nothing to do with what you are asking, I believe. Suffusion supports multiple methods of using thumbnails - the functionality introduced in 2.9, a custom meta field called "thumbnail", an embedded image from the gallery, or an embedded image from an external source. The thumbnail is displayed if you are looking at excerpts.
What you are asking for is something different, akin to a photoblog I think. It is not something you can do directly with the theme since it is not meant for photo-blogging, but you can use a combination of tricks to achieve this without making any changes to CSS or PHP files. Here is how:
1. Go to Suffusion Theme Options -> Blog Features -> Excerpts / Full Contents -> Excerpt Settings for Categories. Set this to display tiles. For the next option set it to display 0 posts with full content.
Mind you, this is not a part of the standard functionality of the theme, so there is a chance you will not get the look you want.