Upload the first photo you posted at the start of this thread to a known URL (and provide us that URL) but in its original state before you did any processing. That way I and anyone else can download it and see what the problem might be.
It sounds to me like your saying that it looks just fine till you resize it down to 600 pixels. If that is where the problem is beginning, then we need to know the exact steps you're using for the resize.
I've been using Save for Web for years and I have always gotten great results from it, so there must be some setting that is being missed that is doing the damage.
Cheers,
rfs
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