Board Thread:Improvements and Issues/@comment-1351982-20131128054130/@comment-44061-20131203223121

This a according to Wiki about he fluid layout. The maximum fluid width for a community's content area is 1600px. A background image that won't be split needs to be even wider than that, in order to be visible on either side of the content. So the "no split" option for an image does not become available until you upload an image that's at least 2100px wide. This will ensure that there's always a decent amount of the background visible on the sides.