Do you have Microsoft Picture Manager? If yes, use it to open the picture. There's an Edit Pictures button at the top of the screen. Click it, and an Edit Picture panel will open on the right side of the screen. Click on Resize. The new panel will let you resize the picture. There are several pre-defined sizes. The Document Large selection will resize the picture small enough to work with the forums. Don't forget to save your changes when you exit.
Hope that helps.