You need a photo editor such as Photoshop, Image-ready, or other program where you can reduce the image size. You can do this via pixels or dimensionally in inches. I would see about either of the first two so you can optimize them for web. I am sure there are others though. Check here for Open Source editors.
http://www.creativebloq.com/photoshop/a ... es-1131641 I do not warrant any of these, as I have not used them. If you do use one, be careful of file extension reassignment during installation.
Have a blessed and prosperous day,
Allen