What are you trying to achieve?
You can not add multiple image textures to a single 'Sprite' node, but you can swap them in gdscript:
var image1 = load("res://icon.png")
var image2 = load("res://test_sprite.png")
$Sprite.texture = image2 # set the texture you want
On the other hand, if you are just trying to make an animation, you have two options:
1 - Use an AnimatedSprite node if your animation frames are spread in a bunch of different images files.
2 - Use a Sprite node and change the "Hframes", "Vframes" and "Frame" properties in the Inspector if you are using a spritesheet.
For more information on 2d animation: 2D Sprite animation