For the blog page, you can choose a variety of layouts, depending on what you want to accomplish.
Page Layout and Sidebars
Find it under "BlogSite Options->Blog Page Options"
A "sidebar" is the section to the left or right of a blog page where you commonly find links, ads, etc. However, to allow your images to be as big as possible it has been common to use no sidebar and allow the blog content to fill the width of the page.
To customize the sidebar, you can add any number of "widgets" by looking in the main menu under "Appearance->Widgets".