I've been running into a problem lately with posterization of my backgrounds when converting to web formats (jpg, png) and its been a bit frustrating. I'll look at my finished image in PS, save for web (Max Jpeg, Bicubic Sharper) and when I look at the jpeg in an image viewer or uploaded to flickr or whatever my once smooth backgrounds have posterized and become blotchy. Looking around the web the only answers I've seen are to add noise. I don't want to add noise, I want smooth backgrounds.
Example:
Greater Yellowlegs by No Small Wave, on Flickr
Unfortunately, flickr only has 1024 or 1600 sizing so I've got to use a smaller image than I would like but the blotchiness is still pretty apparent even at this size. In post the only adjustments I've made to the BG are slight noise reduction (ISO 400, didn't need much), slight tone curve, +15 or so saturation bump, and some gaussian blur. The blur was to try to combat the posterization. It does not look this bad in Photoshop. There is slight posterization but its not really visible unless viewed at high resolution.
So, any advice or tips? Am I missing something when converting to jpeg? Or is it something else?