Sometimes you may be presented with photos that have already been incorrectly saved as GIF files. Your best bet is to open them up in a graphic editing program such as Adobe Photoshop (Windows/Macintosh), Paint Shop Pro (Windows), or the GIMP (Linux) and save them as JPEG files.
You will not get back any quality in your picture, and it will still have a range of only 256 colors (since that's all a GIF can have), but the file size will be smaller, as you can see from the closeups of pictures of Prof. Roberta Stock below. (Recall that the GIF file was 99,488 bytes.)
File size: 36,698 bytes
GIF file, re-saved as JPEG
File size: 40,133 bytes