Problem sorting image columns with tablesorter plugin

Problem sorting image columns with tablesorter plugin

I have a multi column table which has a couple of columns that hold images. The tablesorter plugin works fine for all columns apart from (obviously) the image columns.  I want to be able to sort by these columns too.  One is an urgency/overdue image (a green, gold or red blob) and one a priority image (similar to outlook) .  I'd hoped to be able to use the textExtraction: function to isolate the image column and pass a value in from the alt text property for the images, but have got absolutely nowhere!! The example one from the tablesorter site works fine for my other fields, but I can't get it to trigger on images. I've tried adding the value to sort by infront of the image for the priority column, on the hope that it would sort by this being the first in the column 
 
It doesn't help that my table is a .net gridview!
 
Any help would be greatly appreciated.
 
Chris
 
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