Example :
Convert a 150*100 image into a 150*150 image.
The extra 50 pixels of the height need to be padded with a white background color.

public void ResizeImage(string OriginalFile, string NewFile, int NewWidth, int MaxHeight, bool OnlyResizeIfWider)
{
System.Drawing.Image FullsizeImage = System.Drawing.Image.FromFile(OriginalFile);

// Prevent using images internal thumbnail
FullsizeImage.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);
FullsizeImage.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);

if (OnlyResizeIfWider)
{
if (FullsizeImage.Width <= NewWidth)
{
NewWidth = FullsizeImage.Width;
}
}

int NewHeight = FullsizeImage.Height * NewWidth / FullsizeImage.Width;
if (NewHeight > MaxHeight)
{
// Resize with height instead
NewWidth = FullsizeImage.Width * MaxHeight / FullsizeImage.Height;
NewHeight = MaxHeight;
}

System.Drawing.Image NewImage = FullsizeImage.GetThumbnailImage(NewWidth, NewHeight, null, IntPtr.Zero);

// Clear handle to original file so that we can overwrite it if necessary
FullsizeImage.Dispose();

// Save resized picture
NewImage.Save(NewFile);
}