If you want to squeeze your image a bit more, you could convert your image to Indexed mode. Converting images with smooth color transitions or gradients to indexed mode will often give poor results, because it will turn the smooth gradients into a series of bands. This method is also not recommended for photographs. It'll make the image look bad and grainy.

Figure 6.4.  The indexed image can look a bit grainy. The left image is the wilber in original size, the right one zoomed in by 300 percent.