Use the JPEG Quality slider to set the quality setting to give you a good trade off between image quality and file byte size / download time. This will vary from image to image but you will probably use a figure between 50 and 85. If you have to use a quality higher that 85 it may be worth trying the GIF format. If you are using a setting below 50 you may want to consider resizing or cropping the image instead of compressing it a large amount.
Step 3 (JPEG): add MagiCompression