If you run XP get the Image Resizer here:
This is only part of the solution, also get the Power Toy Calculator from the same site. It's much easier to work with.
Now you need a formula to help you resize pictures accurately to proportion. Try this:
X is the desired width
W is the original width
H is the original height
Y is the proportional height.
Let's say you have a picture to use an an avatar and it's too wide: 200x300. An avatar can be only 160 pixels, so you use this formula, as entered on the calculator:
Now you can enter those dimensions into the Image Resizer.
Here's an example where a picture is too narrow:
Apply the formula using the Power Calculator:
Enter 160 and 219 (always round down) into the Image Resizer and voilà:
Notice that the Image Resizer has taken care of the rounding error for you.
You can use the same formula with some adjustments to correct for height, as well.
[edit on 2005/12/10 by GradyPhilpott]