I go to the Image menu, select Resize/Resample, and enter how large I want the image to be as a percentage of the original, and then save the image. (If you want to keep the original file unchanged, don't forget to change the file name before you save it.)
One feature that the forum does have is that you can post the image as an attachment. That way it shows up as a thumbnail, so it won't blow up the thread no matter how high the resolution. When readers want to see it full size, they just click on the thumbnail.