Assuming you're using Windows, download and install PowerToys from the Microsoft website. In XP, at least, it adds an easy-to-use resizer to the right-click menu. You can resize them to small (640x480) or medium (800x600) and then post them.
The other alternative (and better, really) is to upload them onto a photo-hosting site like Photobucket and then use the built in image commands that Photobucket provides under each thumbnail to link to them in a message. When you link to it, Photobucket automatically resizes it for you.