The generic explanation is that you need to use a graphics program that is compatible with JPG images (the usual format for photographic images). First thing to do is to crop, then resize the image to approximately 600 to 800 pixels wide (or less). Then when you save the image, you can increase the JPG compression ratio (higer compression equals smaller size file, but corresponding loss of quality).
In Windows, you can use the Paint utility. You can also use Paintshop Pro, Photoshop (Elements, CS, Etc.) and a number of other low cost or free utilities. On Macintosh OS X.4 you can use the built-in Preview utility. On Unix systems you can use the free GIMP package.
I'm glad you mentioned GIMP (Graphics Image Manipulation Program, for all you folks who were wondering why they'd name a program so bizarrely), and I'd just like to mention that there is also a version for Windows machines that performs as well as most commercial packages. Takes a bit of getting used to, like all new programs, but I've used it exclusively for a number of years, at home and at work.