Trance Form

I created an avatar by cropping two pictures, putting them on separate layers, lowering the opacity of the top layer, and then flattening the image. The problem is, the file size was 91k, and I needed it below 20k. I then took a screen shot (ctrl+alt+prnt scrn, then paste onto a new image), and cropped it to the original size, and lowered it to 32k. Does anyone have any ideas on how I may further reduce the file size, or remove the middle step all together next time?