Just had a thought - when I uploaded pics yesterday, Specktra resized them so that the maximum dimension was 600 pixels in any direction. If you set the maximum dimension of the picture to be 600 pixels on Irfanview then the photo won't be resized twice which should help preserve the quality better.
To do this go to the Image menu in Irfanview, then go to Resize/Resample and under Set new size simply set the larger of the two existing image dimensions (Width or Height) to be 600 and Irfanview will do the rest. Then save your image.
Please note that Irfanview doesn't automatically save the image you have just edited in the same directory that you opened up the original from so make sure you look to see where the edited version has been saved! This has caught me out lots in the past.