First working solution

ConvertTo16x16.csproj

@@ -35,6 +35,7 @@
   <ItemGroup>
     <Reference Include="System" />
     <Reference Include="System.Core" />
+    <Reference Include="System.Drawing" />
     <Reference Include="System.Xml.Linq" />
     <Reference Include="System.Data.DataSetExtensions" />
     <Reference Include="Microsoft.CSharp" />

Program.cs

@@ -3,13 +3,96 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using System.Drawing;
 
 namespace ConvertTo16x16
 {
+
+    class Count
+    {
+        public int Argb { get; set; }
+        public Point Position { get; set; }
+        public int Anzahl { get; set; }
+
+        public Count(int Anzahl)
+        {
+            this.Anzahl = Anzahl;
+        }
+
+        public Count(int Argb, Point Position, int Anzahl)
+        {
+            this.Argb = Argb;
+            this.Position = Position;
+            this.Anzahl = Anzahl;
+        }
+
+    }
+
     class Program
     {
         static void Main(string[] args)
         {
+
+            //C:\Users\zamk\Bilder\RickNice.jpg
+
+            string bildName = "RickNice.jpg";
+            int PixelAnzahl = 16;
+
+            Bitmap bmp = new Bitmap($@"D:\Pictures16\{bildName}"); // 
+            Bitmap outcome = new Bitmap(PixelAnzahl, PixelAnzahl);
+            int Width = bmp.Width / PixelAnzahl;
+            int Height = bmp.Height / PixelAnzahl;
+
+
+
+            for (int i = 0; i < PixelAnzahl; i++)
+            {
+                for (int j = 0; j < PixelAnzahl; j++)
+                {
+                
+                    List<Count> Argb = new List<Count>();
+
+                    for (int x = Width * i; x < Width * (i + 1); x++)
+                    {
+                        for (int y = Height * j; y < Height * (j + 1); y++)
+                        {
+
+                            bool Exists = false;
+
+                            foreach (var item in Argb)
+                            {
+                                if (bmp.GetPixel(x, y).ToArgb() == item.Argb) //Zu switch case umbauen
+                                {
+                                    Exists = true;
+                                    item.Anzahl += 1;
+                                }
+                            }
+
+                            if (!Exists)
+                            {
+                                Argb.Add(new Count(bmp.GetPixel(x, y).ToArgb(), new Point(x, y), 1));
+                            }
+                        }
+                    }
+
+
+                    Count BiggestItem = new Count(0);
+
+                    foreach (var item in Argb)
+                    {
+                        if (item.Anzahl > BiggestItem.Anzahl)
+                        {
+                            BiggestItem = item;
+                        }
+                    }
+
+                    outcome.SetPixel(i, j, Color.FromArgb(BiggestItem.Argb));
+
+                }
+            }
+
+            outcome.Save($@"D:\Pictures16\16x16{bildName}");
+
         }
     }
 }

bin/Debug/ConvertTo16x16.exe.config

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<configuration>
+    <startup> 
+        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7.2" />
+    </startup>
+</configuration>

obj/Debug/ConvertTo16x16.csproj.FileListAbsolute.txt

@@ -0,0 +1,6 @@
+C:\Users\zamk\source\repos\ConvertTo16x16\bin\Debug\ConvertTo16x16.exe.config
+C:\Users\zamk\source\repos\ConvertTo16x16\bin\Debug\ConvertTo16x16.exe
+C:\Users\zamk\source\repos\ConvertTo16x16\bin\Debug\ConvertTo16x16.pdb
+C:\Users\zamk\source\repos\ConvertTo16x16\obj\Debug\ConvertTo16x16.csprojAssemblyReference.cache
+C:\Users\zamk\source\repos\ConvertTo16x16\obj\Debug\ConvertTo16x16.exe
+C:\Users\zamk\source\repos\ConvertTo16x16\obj\Debug\ConvertTo16x16.pdb