I don't think you can, at least not in the stock camera app. The only resolution settings you have available to you in the stock app are for video. You can slightly lower the picture resolution by telling the phone to shoot in widescreen (6MP) instead of 8MP, but I think it defaults to the 6MP mode already.
The app "Camera Zoom FX" allows you to select different resolutions under the "FX" portion of the settings menu. Camera ICS also allows this. I've been trying out Camera ICS for the past week and like it. It's not as feature ritch as Camera Zoom but it seems to be a lot quicker.