Also, most image hosts have an option for resize directly when you upload the picture from your computer. For example
http://www.tinypic.com is really good. When you upload it you chose which size you want it to be, for example the size called "Webistes/Email" is a very good size for signatures. The "Message Board" size also works sometimes if the image is a bit wider that it's height.
EDIT: Or you can use
http://www.iaza.com for online resizing. No download needed.
EDITEDIT: Or you can use a program called Paint that you'll find under Tools on your computer if you are using a Windows computer.