The way to think about Images and other items on the site is that the site is broken down into 12 columns.
When you place items beside each other there is an automatic 12+12=24px margin between them.
Unless you manually pull on the item everything resizes to fit within those columns and margins. automatically.
In the case of Summary Blocks
This is 1 Column = 960px
This is a 2 Column Break - 468 + 24 + 468 = 960px
This is a 3 Column Break - 304 + 24 + 304 +24 +304 = 960px