Here's how I do it using Paint:
1. Open the picture in Paint.
2. Go to Image, then Stretch/Skew.
3. Enter values under 100 to shrink it down and vice versa. You have to do both the horizontal and vertical, so make sure you enter the same number in each box, or you'll distort the image.
If you go to Image, then Attributes, it'll tell you the size of the picture in pixels. Stretch or skew it until the higher of the two numbers is at or below 500.
4. Save the picture to your hard drive and then attach it when you write a message here.
Hope this helps, I know it can be a bit confusing at first.
-Joe S.