ImageHelper.cs

using System;
using System.IO;
using System.Threading.Tasks;

using SkiaSharp;

public static class ImageHelper
{
    private const double Rotate90 = 90d;
    private const double Rotate180 = 180d;
    private const double Rotate270 = 270d;

    public static byte[] NormalizeImage(byte[] data, int maxSize, double rotation, int quality)
    {
        var source = SKBitmap.Decode(data);

        var factor = Math.Max((double)source.Width / maxSize, (double)source.Height / maxSize);
        var newWidth = (int)(source.Width / factor);
        var newHeight = (int)(source.Height / factor);

        var destination = source.Resize(new SKSizeI(newWidth, newHeight), SKFilterQuality.Medium);
        if (rotation == Rotate90)
        {
            var rotated = new SKBitmap(destination.Height, destination.Width);
            using var surface = new SKCanvas(rotated);
            surface.Translate(rotated.Width, 0);
            surface.RotateDegrees(90f);
            surface.DrawBitmap(destination, 0, 0);

            destination = rotated;
        }
        else if (rotation == Rotate180)
        {
            using var surface = new SKCanvas(destination);
            surface.RotateDegrees(180f, (float)destination.Width / 2, (float)destination.Height / 2);
            surface.DrawBitmap(destination.Copy(), 0, 0);
        }
        else if (rotation == Rotate270)
        {
            var rotated = new SKBitmap(destination.Height, destination.Width);
            using var surface = new SKCanvas(rotated);
            surface.Translate(0, rotated.Height);
            surface.RotateDegrees(270f);
            surface.DrawBitmap(destination, 0, 0);

            destination = rotated;
        }

        using var ms = new MemoryStream();
        destination.Encode(ms, SKEncodedImageFormat.Jpeg, quality);
        return ms.ToArray();
    }
}