With this Unity editor script you can render the camera to an image at any resolution in engine. Should auto fill to be quick and easy. Just throw this in a folder named "Editor" under any directory in the project. To use go to Window/Render Camera. Enjoy

RenderCamera.cs

using System;
using System.IO;
using UnityEngine;
using UnityEditor;

public class RenderCamera : EditorWindow
{
	//Private Variables
	private Camera cam;
	private Vector2 size;
	private string dir;
	private string texName;

	[MenuItem("Window/Render Camera")]
	static void Start()
	{
		RenderCamera win = (RenderCamera)EditorWindow.GetWindow(typeof(RenderCamera));
		win.Show();
		win.First();
	}

	public void First()
	{
		try
		{
			cam = GameObject.Find("Main Camera").GetComponent<Camera>();
		}
		catch { }

		size = new Vector2(1920, 1080);
		dir = "Screenshots/";
		texName = "Render";
	}

	void OnGUI()
	{
		#region Options
		cam = (Camera)EditorGUILayout.ObjectField("Camera", cam, typeof(Camera), true);
		size = EditorGUILayout.Vector2Field("Size", size);

		//Alot of code for that single line of input but it needed to be done...
		EditorGUILayout.BeginHorizontal();

			GUILayout.Label("Directory");

			Color temp = GUI.color;
			GUI.color = new Color(temp.r, temp.g, temp.b, 0.5f);
			GUILayout.Label("Assets/");
			GUI.color = temp;

			dir = EditorGUILayout.TextField(dir);

			texName = EditorGUILayout.TextField(texName);

			GUI.color = new Color(temp.r, temp.g, temp.b, 0.5f);
			GUILayout.Label(".png");
			GUI.color = temp;

		EditorGUILayout.EndHorizontal();
		#endregion

		#region Render
		if (GUILayout.Button("Render"))
		{
			//Create Stuff
			RenderTexture rt = new RenderTexture(Convert.ToInt32(size.x), Convert.ToInt32(size.y), 0);
			Texture2D tex = new Texture2D(rt.width, rt.height);

			//Snap
			cam.targetTexture = rt;
			cam.Render();

			//Read
			RenderTexture.active = rt;
			tex.ReadPixels(new Rect(0, 0, rt.width, rt.height), 0, 0);
			tex.Apply();

			//Clean Up
			cam.targetTexture = null;
			RenderTexture.active = null;

			//Save To File
			byte[] b = tex.EncodeToPNG();
			File.WriteAllBytes(Application.dataPath + "/" + dir + texName + ".png", b);
		}
		#endregion

		#region Help
		EditorGUILayout.HelpBox("16:9:\n1080p: 1920x1080\n1440p: 2560x1440\n4k: 3840x2160\n\n21:9:\n1080p: 3440x1080\n1440p: 2560x1440\n4k: 5120x2160", MessageType.None);
		#endregion
	}
}