One of my programs does a similar task, it creates thumbnails to a
standard size, where the longest dimension of the thumbnail is fixed.
There is a certain amount of other code to check that it does not
create thumbnails of thumbnails (it is from a batch process that deals
with groups of files).  Careful with line wrap, this is cut and pasted
from my code and was not formatted for usenet.

rossum

// ==== Code Begin ====

const double thumbSize = 175.0;  // Longer dimension of thumbnails

/// <summary>
/// Creates a thumbnail of an image file in the same directory.
/// </summary>
/// <param name="jpeg">The image file from which to create the
thumbnail.</param>
/// <returns>True if a thumbnail was created, false
otherwise.</returns>
private bool makeThumbnail(FileInfo jpeg) {
   StringBuilder thumbName = new StringBuilder(jpeg.FullName);
   thumbName.Replace(m_targetExtn, m_thumbExtn);
   if (File.Exists(thumbName.ToString())) {
       MessageBox.Show("Warning: " + jpeg.Name + " thumbnail already
exists.",
                       "Thumbnail Batch",
                       MessageBoxButtons.OK,
                       MessageBoxIcon.Warning);
       return false;
   } // end if

   // Check not already a thumbnail
   if (jpeg.Name.Contains(m_thumbExtn)) {
       return false;
   } // end if
   Image img = Image.FromFile(jpeg.FullName);
   if (img.Height <= thumbSize || img.Width <= thumbSize) {
       img.Dispose();
       return false;
   } // end if
   
   // Scale the thumbnail
   int newWidth, newHeight;
   if (img.Height > img.Width) {
       newHeight = (int)thumbSize;
       newWidth = (int)(img.Width * thumbSize / img.Height);
   }
   else {
       newWidth = (int)thumbSize;
       newHeight = (int)(img.Height * thumbSize / img.Width);
   } // end if

   Image thumb = img.GetThumbnailImage(newWidth, newHeight, null,
(IntPtr)null);
   thumb.Save(thumbName.ToString());
   img.Dispose();
   thumb.Dispose();
   return true;

} // end makeThumbnail()

// ==== Code End ====

Here's a ready to use method I wrote for resizing pictures and maintaing
their aspect ratio:
http://www.geekpedia.com/code7_Method-for-resizing-pictures-with-aspect-ratio.html

It works with various formats.

Good luck!
Andrew