If you use a computer that operated with Windows XP, Microsoft offers a simple PowerToy called Image Resizer. You can download it here, for free:
Once downloaded, double click on the file to open the program. It will install quickly after a few clicks of the mouse.
Find the images on your computer you'd like to resize. Simply right click over the image, and the new option to Resize Pictures will appear in the pop-up menu.
Note: If you want to resize many at once, just highlight and select them all, then right click and click on Resize Pictures.
Choose the size you would like. I suggest 640 x 480, which is the size I normally use to share all of my images.
If you click on Advanced, there are a couple of extra options available to you, so take a look to see if they are of value to your needs.
Once you click OK, it will proceed to make a new image at the size you've selected, and it will have a new filename. The original will not be overwritten unless you've set the program to do so. Below you can see how the original image is still in the directory, and the new one is pointed out as the newest addition.
This is a great tool for those that need a quick resizer and don't want to load up any programs to do so. It is always available in the pop-up menu. Now when you share your images online, they won't be too large and will fit people's monitors when they view them.
Additional options that are webbased: