It sometimes happens when images are dragged to the desktop.
If you control-click and choose 'save image to' or 'save image as' it shouldn't happen.
But this is nothing new!
Safari: Downloaded file's filename may have an additional extension
http://support.apple.com/kb/TA24293
That's from June 11, 2007!
Working from my defective memory, this happened to me, but stopped if I unchecked 'Open safe files after downloading' in Safari Preferences/General.
At least I think that's how I stopped it - it hasn't happened since!