Debbie, I forgot to say that to be able to "save for web" you need Photoshop or Illustrator - there may also be other programmes where you get that option, but they're the ones I know of.

Once you've worked on a picture (cropped, resized etc.), you choose "save for web" instead of just "save as". We've set ours up to show 4 different compressions in one window, and you can choose which one you want to use. They are 4 different sizes.