An easy and free little program for resizing your photos for posting on the internet can be downloaded from Irfanview:
http://www.irfanview.com/

This is a handy program, highly recommended by Kim Kommando, so you can be pretty sure that it's safe and effective. On the other hand, you might already have a graphics or photo editing program on your computer, like Photoshop, Photoshop Elements, Paint Shop Pro, or Digital Image Pro (that's the Microsoft one). These are not as easy to use as the Irfanview product, but if you already have one, you might as well use it. Most of them have a "Save for Web" feature that walks you through the steps of resizing an image for posting on the internet.