新建.net 6控制台项目
安装System.Drawing.Common包

代码如下

using System.Drawing;
using System.Drawing.Imaging;

namespace PngCompress02
{
    internal class Program
    {
        static void Main(string[] args)
        {
            CompressPngImage(@"E:\Desktop\6.png", @"E:\Desktop\6_60.png", 60);
            Console.WriteLine("完成");
        }

        public static void CompressPngImage(string sourcePath, string targetPath, long quality)
        {
            using (var image = Image.FromFile(sourcePath))
            {
                var encoderParameters = new EncoderParameters(1);
                encoderParameters.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);

                var imageCodecInfo = GetPngImageCodec();
                //var imageFormat = ImageFormat.Png;
                Bitmap bitmap = new Bitmap(image);
                bitmap.Save(targetPath, imageCodecInfo, encoderParameters);
            }
        }

        private static ImageCodecInfo GetPngImageCodec()
        {
            var codecs = ImageCodecInfo.GetImageEncoders();
            foreach (var codec in codecs)
            {
                if (codec.FormatID == ImageFormat.Png.Guid)
                {
                    return codec;
                }
            }
            throw new ArgumentException("No PNG image codec found.");
        }
    }
}

测试结果

既要压缩图片也要改变图片大小

实现思路：先改变图片大小后压缩处理

/// <summary>
/// 重新改变png图片的大小
/// </summary>
/// <param name="image"></param>
/// <param name="newWidth">改变成为的宽度大小</param>
/// <param name="newHeight">改变成为的高度大小</param>
/// <returns></returns>
public static byte[] ResizePngImage(Image image, int newWidth, int newHeight)
{
    // 确保图片格式为Png  
    //if (image.RawFormat.Guid != ImageFormat.Png.Guid)
    //{
    //    throw new InvalidOperationException("不支持的图片格式。请使用PNG格式。");
    //}
    MemoryStream memoryStream = new MemoryStream();
    // 创建一个新的Bitmap对象，并设置其分辨率为原始图像的分辨率  
    using (Bitmap newImage = new Bitmap(newWidth, newHeight))
    {
        newImage.SetResolution(image.HorizontalResolution, image.VerticalResolution);

        // 使用Graphics对象从原始图像绘制到新的Bitmap上  
        using (Graphics graphics = Graphics.FromImage(newImage))
        {
            // 设置高质量插值法以进行图像缩放  
            graphics.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;

            // 在新图像上绘制原始图像  
            graphics.DrawImage(image, new Rectangle(Point.Empty, new Size(newWidth, newHeight)));
        }
        // 对于PNG图像，我们需要特别处理透明度。这里我们保存带有透明度的图像。  
        newImage.Save(memoryStream, ImageFormat.Png);
    }
    return memoryStream.GetBuffer();
}

//压缩图片，返回图片byte[]数组 宽度高度和质量
public byte[] compressPngImage(int sW,int sH int flag)
{
	var iSource = System.Drawing.Image.FromFile(sFile);
	var pngStream = ResizePngImage(iSource, sW, sH);
	var pngImage = Image.FromStream(new MemoryStream(pngStream));
	var encoder = ImageCodecInfo.GetImageEncoders().FirstOrDefault(c => c.FormatID == ImageFormat.Png.Guid);
	if (encoder != null)
	{
	    // Set the quality parameter for the PNG encoder  
	    var encoderParameters = new EncoderParameters(1);
	    encoderParameters.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, flag);
	    // Save the resized image using the PNG encoder with the specified quality level  
	    var thumbnailStream = new MemoryStream();
	    pngImage.Save(thumbnailStream, encoder, encoderParameters);
	    return thumbnailStream.GetBuffer();
	}
}