Without overclocking there is quite a difference in clock speeds. Since the image editing bench uses single threaded GIMP effects the 4790k with its max. turbo of 4,4Ghz is faster than the 3.6GHz of the 5820k even though it's 4 cores vs. 6.
EDIT: Looking at the FSB speed it seems to be overclocked already, can you verify clock speeds when the bench is running? The difference even at 3.6 vs. 4.4 should not be that great in image editing.