How do I shrink an image to be usable as an avatar? 
Get a capable graphics software application (I use Jasc Paint Shop Pro, but there are many, many others). Resize to suit.
Note that, depending on the type of image you are starting off with, you may find that it turns to junk when shrunk to the 100 x 100 pixel (20 KB max) size required by this forum. Stiff, angular images with lots of straight lines do poorly, even with features like anti-aliasing employed in the shrinking, if the difference in size is too great. Colour images seem to do better.
Have fun!
djm