TIFF is an acronym for Tagged Image File Format - a format for storing image data. See 
http://www.libtiff.org/
 for the source code.
See 
Image, 
Img, 
Photo format - HPL (Fitzhugh), and 
QuickTimeTcl for some Tk extensions which support TIFF.  See also 
tiff package (in pure tcl), 
Xbit and 
tcllib.
ZLM: this is my summary of what each package provides. Authors, please correct me. Who wrote 
tiff package (in pure tcl) anyway?
tiff package (in pure tcl) - this allows creating Tk image (type photo) objects from TIFF images (compression modes supported: Uncompressed,  CCITT 1D, Group 3 Fax, Group 4 Fax, LZW, JPEG or PackBits.) ; saving uncompressed RGB TIFF files.
img extends the existing Tk command image so that it supports TIFF files. Can write compressed files (compression modes: uncompressed, jpeg, packbits, or deflate). Contains the package 
tifftcl, which is a Tcl wrapper around the TIFF C library.
Xbit  also supports multi-image files.
tcllib tiff - read and write image metadata such as size and comments, read and write uncompressed images.