I think you can do it with a Camera and a script that moves across the image (which would be a Sprite):
extends Camera
var _dragging = false
func _ready():
set_process_input(true)
func _input(event):
# Note: on mobile, Godot emulates mouse events,
# so it should work on desktop too without changes.
if event.type == InputEvent.MOUSE_BUTTON:
_dragging = event.pressed
elif event.type == InputEvent.MOUSE_MOVE and _dragging:
var motion = Vector2(event.relative_x, event.relative_y)
set_pos(get_pos() + motion)