Exam images, with the explanation
Biliary tract and gallbladder: distended gallbladder with a stone impacted in the neck, a thickened wall, pericholecystic fluid and a positive sonographic Murphy's sign.
The stone is a hyperechoic (white) image inside the gallbladder, with a black acoustic shadow behind it, because the ultrasound cannot pass through it. Here it does not move when the patient changes position: it is impacted in the neck and blocks emptying, and indeed the gallbladder is distended.