I think the forum software is compressing the images for the preview size and deminishing the quality in the process. However I feel like some images suffer more from it than others, but I've seen pretty devastating examples as well, where I thought some image had significant IQ flaws or would need some serious color-corrections, and when I opened the original uploads in Photoshop it became clear that it was just due to that compression. Perhaps it's an auto conversion to a different colorspace or some additional JPEG compression, but it did look pretty bad in some cases.
EDIT: Here's my attempt at showing the difference:

Full-size link: live.staticflickr.com/65535/53295095431_71b12a8162_o.jpg
I'm aware that there's some Browser-specific color-management in place which can be altered, but my images look pretty comparable to my Photoshop previews on flickr (at least they don't stray in the same way as here), so I don't want to adjust my Browser just in order to have preview images on dprevived match up...