GeneratePictureFrame.cs

using System.IO;
using UnityEngine;

public class MaterialGenerator : MonoBehaviour
{
    public Texture2D imageTexture; // Assign in Inspector
    private Material generatedMaterial;

    void Start()
    {

        //Get raw size of image
        string assetPath = UnityEditor.AssetDatabase.GetAssetPath(imageTexture);
        byte[] tmpBytes = File.ReadAllBytes(assetPath);
		imageTexture.LoadImage(tmpBytes);
        float width = imageTexture.width;
        float height = imageTexture.height;

        if (width>height){
            float x = (width/height)*gameObject.transform.localScale.x;
        gameObject.transform.localScale = new Vector3(x, gameObject.transform.localScale.y, gameObject.transform.localScale.z);
        } else {
            float z = (height/width)*gameObject.transform.localScale.z;
          gameObject.transform.localScale = new Vector3(gameObject.transform.localScale.x, gameObject.transform.localScale.y, z);  
        }
        
        Shader shader = Shader.Find("Universal Render Pipeline/Lit");
        generatedMaterial = new Material(shader)
        {mainTexture = imageTexture};

        Renderer renderer = GetComponent<Renderer>();
        renderer.material = generatedMaterial;
    }
}