The GenerateThumbnail() C# function resizes pictures for various formats, such as JPEG, PNG and BMP, and maintains their aspect ratio.

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using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;
 
public void GenerateThumbnail(string filePath, string origFileName, string thumbFileName, ImageFormat imgFormat)
{
    System.Drawing.Image origImage = System.Drawing.Image.FromFile(filePath + @"\" + origFileName);
 
    float WidthPer, HeightPer;
    int NewWidth, NewHeight;
 
    if (origImage.Width > origImage.Height)
    {
        NewWidth = ThumbnailSize.Width;
        WidthPer = (float)ThumbnailSize.Width / origImage.Width;
        NewHeight = Convert.ToInt32(origImage.Height * WidthPer);
    }
    else
    {
        NewHeight = ThumbnailSize.Height;
        HeightPer = (float)ThumbnailSize.Height / origImage.Height;
        NewWidth = Convert.ToInt32(origImage.Width * HeightPer);
    }
    System.Drawing.Image origThumbnail = new Bitmap(NewWidth, NewHeight, origImage.PixelFormat);
    Graphics oGraphic = Graphics.FromImage(origThumbnail);
    oGraphic.CompositingQuality = CompositingQuality.HighQuality;
    oGraphic.SmoothingMode = SmoothingMode.HighQuality;
    oGraphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
    Rectangle oRectangle = new Rectangle(0, 0, NewWidth, NewHeight);
    oGraphic.DrawImage(origImage, oRectangle);
    origThumbnail.Save(filePath + @"\" + thumbFileName, imgFormat);
    origImage.Dispose();
}