It is very easy...
Paint Shop Pro (Jasc Software - http://www.jasc.com/products/psp/
) is relatively inexpensive and is more than what most people need. Don't waste your money on Photoshop by Adobe.
In Paint SHop Pro, when saving your images as jpeg (.jpg) you have compression options. At 10% compression, the quality is darn near the same. At 15 to 20 %, looking closely, you will see some minor lose of quality. On some images, you can get away with up to about 25% compression (which can cut the file size in half).
BTW, if you resize your image 25% smaller, the file size can be up to nearly 50% less (depending on the image).
For a FREE quick method, you can "crunch" them online at http://www.spinwave.com/crunchers.html
Or even better, http://www.jpegwizard.com