Answer:
Change the pixel dimensions by cropping or resizing, or change the number of colors, or quality settings, or use a different file format. The best way is if you have Photoshop or Photoshop Elements, use the "save for web" option. It gives you full control over all these options in one easy to use window. Find it next to the save option in the file menu. It will tell you the current file size of the options you choose before you save, and show you what the image will look like so you can reduce size as far as possible without making the quality too bad.