OP, I am also short and definitely not at my skinniest right now.
During the last set of headshots I had taken, I had a few different tops to use. If they'll allow you to have a few different tops, that would be great. The thing that ended up looking the best was a very simple black top with flattering neckline. I had on a statement necklace and some simple earrings.
Those photos were the best of the batch. Just remember that no one is actually really going to be able to see the top that well, EG what it's made of. I think you want a shape and a silhouette that is going to flatter you.
(I felt pretty good about these pictures when one of my colleagues gave me some unsolicited feedback and told me I looked "expensive"... Like a high end executive.