Find it under "BlogSite Options->Blog Page Options"
On the main Blog Page you have a choice between showing the full content of each post or only showing a brief excerpt.
The full content would show all the pictures and anything you put into the post. However, with 10 posts going down the page, it can slow down the page load.
By showing only the excerpts, you get a clean fast loading page with a quick overview of the blog posts.
*To edit the number of blog posts per page, go to Dashboard-->Settings-->Reading and then change the number in the box after "Blog pages show at most __ posts"