Although you can do this in an image editor such as Photoshop, Aviary, Gimp, or Paint,  one tool that can make this process very simple is called Image Optimizer.  http://www.imageoptimizer.net/Pages/Home.aspx

With this tool, all you need to do is right click on the image file and select Optimize Here.  The program will create a new web optimized version of your photo which is almost always under 100k.  If it's not, the program's advanced settings will be able to do that.