Here are some links to the original file and the produced thumbnail.
In Firefox, if you right click the image and click properties, the original is listed as type JPEG with a size of 134.15 KB (137371 bytes) and the thumbnail is type BMP with a size of 213.5 KB (218624 bytes).
The source of the original image is the NASA Earth Observatory. The article is here:
And the original link is here:
I picked the image at random to test the thumbnailing feature and other image handling features. I suspect the problem is specific to this image, but I don't know what about the file would cause the thumbnailer to encode it as a BMP instead of JPEG.