TableSorter: Use SVGs with PNG fallbacks instead of GIFs
Also removes the never-used sort_none.gif.
You have changed the content of the files "core\resources\src\jquery\images\sort_down.*" and "...sort_up.*". The sort_down.gif was former a triangle upwards! This symbol show the actual sort state (upwards) and after click table sorts down.
Please change the filenames.