This tutorial is to help you understand how to resize images using
Windows' Paint program. If you have a different graphics program that you
already know how to use, I recommend using it. :) (Paint is not the
easiest graphics program to work with!) But Paint comes with Windows, so if you don't have another
graphics program, these instructions should help you with resizing an
image in Paint. There is an alternative method shown on YouTube, and
that video is included at the end of this post.
If you have
an image that needs to be resized for your avatar, open the Paint
program by going to your Start menu -> All Programs -> Accessories
-> Paint. After the program is open, click on File -> Open, and
browse to where your image is on your hard drive. Select the image and
click Open.
Once the image is open, click on your Select tool shown in the screen shot below.

Then
select the portion of your image that you want to use. You may select
just a part of the image. But if it is the whole image you want, then
click at the top corner and drag the box to the opposite bottom corner.

You
should now be able to locate "handles" at the corners and along the
sides, top and bottom of the selection. (The "handles" are quite small
in Paint, but they
are there.)
:P Locate the handle on the bottom right corner and drag the
selection, making it smaller. While holding the mouse button and
dragging the selection, you will see at the bottom right corner of the
program window some numbers. (See screenshot below.) These are the
pixel dimensions of your selection. For resizing your avatar, you will
need to drag the selection box until the numbers show about 100x100.
Of
course, if your image is a rectangle instead of square, you'll need to
make a rectangle (something like 120x80), but as long as it's under
150x150, it should be fine.

Now you probably have a teeny, tiny image where your larger image used to be. :)

Now,
click on Image -> Attributes. A dialogue box will appear with
places for you to specify width and height. If you remember the numbers
at the bottom right of the screen when you resized the selection, type
those beside width and height. If you don't remember them exactly, type
100 beside both width and height and click OK.

Be sure to save your image where you can find it and upload it when needed!
Also, make sure you use the "Save as"
option when saving the pic. If you use "Save" it will automatically
save over the original larger pic. Don't want to lose that! Use "Save as" to give the resized pic a
new file name.
Alternative method as shown on YouTube: