the easiest way to do this is to put it in Photoshop, click on "Image" then go to "image size" change it to 100x100. then go to "file" and go to "save for web and devices". then use that image. If you cant figure it out send me the file, i'll convert it and send it back to you.
Yup, that's the easiest way, but you gotta have photoshop tho
I use photobucket to resize my photos. Obviously photoshop is going to be a better program but if its just for something like your avatar, photobucket works just fine