That was easy... Hardly took a minute...
Right click on the image's icon.
Click on "Edit" then.
The picture will open in MS Paint.
If you have Windows 7 (new MS Paint) you can see "Rotate" option on the main toolbar. Click on "Flip horizontal".
If you have older version of Paint, press Ctrl+A to select all and then right click on the image. Go to "Rotate" and flip horizontally.
Save the file and you are good to go.