Updates to 'Create and manage images' section in Admin User Guide

Updates to the Admin User guide section 'Create and manage images' in the
dashboard chapter to include the changes to the fields in the 'Create Image'
window.
Partial-Bug: #1279133

Change-Id: I079543d4d47f3660bb18435067cc44ff178bb136
Merge "Updates to 'Create and manage images' section in Admin User Guide"