I tried to delete an image from an item and found out that all image files were deleted from media/k2/items/src and media/k2/items/cache (ie, the original image and all other resized images, like _XS, _S, _M, _L, _XL) but not the _Generic one!!!
to fix this issue just add the following in administrator/components/com_k2/models/item.php before line 287 in function save()
if (JFile::exists(JPATH_ROOT.DS.'media'.DS.'k2'.DS.'items'.DS.'cache'.DS.$filename.'_Generic.jpg')) {
JFile::delete(JPATH_ROOT.DS.'media'.DS.'k2'.DS.'items'.DS.'cache'.DS.$filename.'_Generic.jpg');
}
