It would be convenient to be able to specify default values for SortAscImageURL and SortDescImageURL at a level above the individual column elements

At present the SortAscImageURL and SortDescImageURL attributes appear in the GridBoundColumn element.  This requires specifying them for each column definition.  Often the UI will use the same icons for all sort-able columns.
An obvious choice might be the <SortingSettings> element, as is done for the sorting tool tip and sorted background color.