2 ways:
1 upload the picture to photobucket. then hit "edit" and resize it to the message board size. then copy the IMG file and paste it to your post.
2) resize the picture and then hit manage attatchment at the bottom of the post and upload the pic. it has to be smaller or equal to 500x500 though
use photobucket its free. dont have to edit anything just click on img tag and it automatically copies link and u paste in in quick reply box or message box.