I don't have an actual thumbnailer, but I use a "thumb" command to let people post pics at a size I setup (300x400) on my board and when you click on the pic the fullsize version opens in a new window. You can see a demo here...
click here I'm not sure if this is what you're after, and it doesn't maintain the original aspect ratio. I just wanted a way to clean up threads with large images in them.
