What is configuration image zero and one?
From page 2-3:


  Note: In dual compressed images mode, you can use the CONFIG_SEL pin to select the configuration image


The table on page 2-4 shows the Flash Memory Sectors for multiple Internal Configuration Modes. The first mode (dual compressed image) has a "Compressed Image 1" and a "Compressed Image 0".

When using any of the other 4 modes, the CONFIG_SEL pin connection is irrelevant. Page 3-6 describes the step-by-step to select a different configuration mode.

You should probably want to refer to the tables that start on page 3-18 (pg 50) about image selection. There you can see how CONFIG_SEL pin changes the image to be loaded (from image0 to image1) on "DUAL" configuration image mode.