To reproduce:

1) Open project
2) Open Temp scene
3) Start scene, click left mouse button and hold
4) If the image starts jumping from one side of the screen to the other, then make the width of the game window smaller and start it again
5) Repeat step 4 until, when the mouse button is pressed and moved, the image moves smoothly
6) Notice, that the Unity icons move differently from the background image (they shake a bit, even though they should appear as part of the background)

Notes: the Unity icons have different z values, however the issue remains when they have the same z value as the background image.