Resizing an Image is very important on the web.
You reduce loading times on a website and the same time preserving the quality on the image. The larger the image the longer it will take to load and thus it is important to resize it. Google now will penalize your site if it loads slowly and one of the recommendation is to optimize the images (reduce the size while maintaining the quality)
One free desktop tool I use for this is Pixlr which can be downloaded from Pixlr.com . If you are looking to resize images quickly without the extra fluff and optimize them for the web this is a great tool.