You are correct the trapezium is the taller bone and you inject between the scaphoid and the trapezium.
Distal to this you can see the 1st CMC joint.
I think when we do this the transducer is placed slightly more to the ulnar side so you see the FCR tendon in long axis and the scaphoid and trapezium bones. To then show the 1st CMC you slide the transducer slightly radially and distally.
If you get the needle between the scaphoid and trapezium you will be in the STT joint.
Steve