there should be settings to set sample size and/or pixel depth.
or open pic in another program, corel photopaint, fireworks, photoshop etc and re canvas, color depth or pixel size there to make a pic the size you need with smallest file size.
i use fireworks for all my stuff as its made just for web graphics and does it in a sec...... you can export with multiple views to see quality and size of all the different ways you can change it.