Unexpected Thumbnail Generation Behavior

[Cecil Windham]

I'm enabling users to rotate photos and generate thumbnails.

I have discovered that if, after a user rotates a photo 180 degrees, then
generates a thumbnail, that the resulting thumbnail appears unrotated (i.e.,
in original position, or rotated zero degrees).

Why is this? (and yes, I have verified that the "original" from which the
thumbnail is generated has, in fact, been rotated 180 degrees prior to
generating the thumbnail).

Here is the code that creates thumbnail:
System.IO.FileStream fs = new System.IO.FileStream(pathToFullSizeImage,
System.IO.FileMode.Open, System.IO.FileAccess.Read);
System.Drawing.Image objThumbnail =
System.Drawing.Image.FromStream(fs).GetThumbnailIm age(iWidth, iHeight, null,
IntPtr.Zero);

Here is the code that rotates the original:
System.IO.FileStream fs = new System.IO.FileStream(pathToOriginal,
System.IO.FileMode.Open, System.IO.FileAccess.ReadWrite);
System.Drawing.Image imageToFlip = System.Drawing.Image.FromStream(fs);
imageToFlip.RotateFlip(System.Drawing.RotateFlipTy pe.Rotate90FlipNone);
// In order to rotate 180 degrees, the user clicks a button twice, which
executes above code twice.

Repeatedly clicking the button that executes the above code causes the
original to rotate in 90 degree increments: can go all the way around with
no problem, and thumbnails can be subsequently generated that reflect the
original's current rotation position - with the only exception being when
the original is rotated 180 degrees.

Any clues or explanations are greatly appreciated!