You can re-size pictures several ways. Photobucket has a re-sizing tool, or you can use Picasa (or similar free software).
Additionally, and like most people choose to do, you can save a duplicate picture of one you already have saved on you PC. Since you'll be changing the size of your original picture, save it again. One to keep in it's original format/size and one to down-size for the forum. Once a duplicate is saved, open one of them and then "edit" it. In this process, you can manually select the width x height to those of the forum requirements, as well as other modifications as dictated by your individual image viewer.
wannago, if you need individual help, contact a Moderator (or myself) by sending a personal message for assistance.