What format is the file saved in? The best one to use is JPG, which uses compression to cut down the size, usually without much obvious loss of quality. Possibly the worst to use is Windows Bitmap (BMP) which is not only large, but also can't be opened by several browsers.

Most image editing programs can save a file as a JPG; many will also have the ability to resize and/or crop the picture to cut down the file size even more.