Open in your image editing software (chances are good that your camera came with some basic application to do this). Save it under a different name. Look through the menu for "resize" or "resample". Select the size you want. Doing so by pixels is usually better than by inches. I would not make them any larger that 1024x768. 800x600 is a good compromise size. Make sure resolution is set to 72dpi. Anything higher than that is not needed for posting on the web. Resave (you arleady renamed it, so your original file is still intact). If you're saving it as a .jpg, and your image editing software gives you the option, set compression to 10% (optimized). That should give you a pretty good looking image of reasonable size.
If that isn't clear, drop me a PM and I can try to help you out more.