The ScaleTool we wrote for HoloLens doubled as a teaching example of the GestureRecognizer, and thus made things more complicated than they should be. For the mobile version, we'll just rewrite the script so it's much more similar to the MoveTool one. Here is the whole thing:

File: ScaleTool.cs
using UnityEngine; 
 
public class ScaleTool : MonoBehaviour { 
    private PictureController picture; 
    private bool isEditing = false; 
    private Vector3 originaButtonScale; 
 
    //Used to calculate the mouse position 
    private Vector3 startPosition = Vector3.zero; 
    private Vector3 currentPosition = Vector3.zero; 
    private Vector3 initialScale = Vector3.zero; 
 
    void Start() { 
        picture = GetComponentInParent<PictureController>(); 
        originaButtonScale = transform.localScale; 
    } 
 
    void Update() { 
        if (isEditing) { 
            currentPosition = Input.mousePosition; 
            float difference = (currentPosition - startPosition).magnitude; 
            //Scaling down is possible by dragging your mouse to the left. 
            int direction = currentPosition.x > startPosition.x ? 1 : -1; 
            float scaleFactor = 1 + (difference / Screen.width) * direction; 
            if (scaleFactor > 0.1f) { 
                picture.transform.localScale = initialScale * scaleFactor; 
            } 
        } 
        if (!Input.GetMouseButton(0)) { 
            DoneEdit(); 
        } 
    } 
 
    private void OnMouseDown() { 
        if (!isEditing) { 
            BeginEdit(); 
        } 
    } 
 
    public void BeginEdit() { 
        if (!isEditing) { 
            isEditing = true; 
 
            transform.localScale = originaButtonScale * 2.5f; 
            startPosition = Input.mousePosition; 
            initialScale = picture.transform.localScale; 
        } 
    } 
 
    private void OnMouseUp() { 
        if (isEditing) { 
            DoneEdit(); 
        } 
    } 
 
    public void DoneEdit() { 
        if (isEditing) { 
            isEditing = false; 
            transform.localScale = originaButtonScale; 
        } 
    } 
} 

Now we just need to change the MoveTool script.