go to question bank→ create a question of type "Drag and drop onto image"， using either jpg or png →save and continue to edit→ the draggable pictures not showing properly but some small dots, which are hard to be found but can be drag go adjust their coodination. → save and review → it shows normal. but when drag the items into their target zones, they shrink small as small squares.
Previously it was normal when I use Moodle 3.8 earlier, don't know what caused the change.