in a graphics program to do this, and even then your ability to enlarge the image will depend on its format.
If you've saved it from a web site it will be a JPEG or Gif, and in both cases enlarging the image size will result in a loss of quality. Web images have been compressed, and are usually quite small physically. They don't enlarge at all well, although you can try.
If your image is a bitmap you'll be OK.