2003.10.01 04:08 "[Tiff] Reading thumbnail", by Marco Schmidt
By reading TIFF Specification, full-size image is in IFD0 and thumbnail is in IFD1.
No, not really. Sometimes a thumbnail gets stored as a second image in the TIFF file. Sometimes there is no thumbnail. Sometimes there is a second image, but it's not a thumbnail version of the first image.
So I'd be careful making assumptions about what you can find in a TIFF file.