نقل قول://Namespace Reference
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;

/// <summary>
/// method for resizing an image
/// </summary>
/// <param name="img">the image to resize</param>
/// <param name="percentage">Percentage of change (i.e for 105% of the original provide 105)</param>
/// <returns></returns>
public Image Resize(Image img, int percentage)
{
//get the height and width of the image
int originalW = img.Width;
int originalH = img.Height;

//get the new size based on the percentage change
int resizedW = (int)(originalW * percentage);
int resizedH = (int)(originalH * percentage);

//create a new Bitmap the size of the new image
Bitmap bmp = new Bitmap(resizedW, resizedH);
//create a new graphic from the Bitmap
Graphics graphic = Graphics.FromImage((Image)bmp);
graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
//draw the newly resized image
graphic.DrawImage(img, 0, 0, resizedW, resizedH);
//dispose and free up the resources
graphic.Dispose();
//return the image
return (Image)bmp;
}