Col,
Just some further information. I checked the embedded profile in both images in your
Sun on the Sand thread and they are tagged with AdobeRGB.
When I opened both files, I got this dialog box (which is for the first of your images):
I would almost bet that your color settings in camera are set to AdobeRGB and that in your color settings in Photoshop, under Color Management Policies, you haven't got "Ask when opening" checked next to 'Profile Mismatches'.
As a result, you have told Photoshop to override your color setting and use the image profile without warning you. So, Photoshop is using AdobeRGB encoding and then when you save the file out and upload it, you see a huge difference in quality because AdobeRGB is not a good match for the screen unless you are using color managed software.
Regards,
Peter