ImageWithBytePacker.cs

public interface IBytePacker<TColor> 
  where TColor: struct, IPackedPixel<TPacked>
{
  // Better with Span<TColor> and Span<byte> if possible!
  // Even better with a sourceRectangle parameter instead of sourceOffset
  void PackBytes(TColor* source, byte[] dest, int destOffset, int count, ComponentOrder componentOrder);
  
}

#if SOLUTION_1
// Image with 3 Type parameters:
public abstract class ImageBase<TColor, TPacked, TBytePacker> : IImageBase<TColor, TPacked>
        where TColor : struct, IPackedPixel<TPacked>
        where TPacked : struct,
        where TBytePacker : IBytePacker<TColor>
{
  // returns a packer instance:
  public TBytePacker Packer => new TPacker();
}
#endif // SOLUTION_1

#if SOLUTION_2
// TColor implements IBytePacker 
// Image has only 2 Type parameters, but confusing
public abstract class ImageBase<TColor, TPacked, TBytePacker> : IImageBase<TColor, TPacked>
        where TColor : struct, IPackedPixel<TPacked>, IBytePacker<TColor>
        where TPacked : struct
{
  // returns a packer instance:
  public TColor Packer => new TColor();
}
#endif SOLUTION_2
  
// Sample implementation:

public struct Bgr565 : IPackedPixel<ushort>, IEquatable<Bgr565>
#if SOLUTION_2
  ,IBytePacker<Bgr565>
#endif
{
  ...
  #if SOLUTION_1
  public struct Packer : IBytePacker<Bgr565>
  {
     // This struct contains no members only, the implementation
     void PackBytes(Bgr565* source, byte[] dest, int destOffset, int count, ComponentOrder componentOrder) {}
  }
  #endif
  
  #if SOLUTION_2
  void PackBytes(Bgr565* source, byte[] dest, int destOffset, int count, ComponentOrder componentOrder) {}
  #endif
}