tiff to tns conversion seems to represent attempts in transferring picture files (.tiff, .gif, .png etc.) to TI-Nspire product line of graphing calculators developed by Texas Instruments, which use .tns documents. Possibly only certain pixel sizes of the pictures can be transferred, so you may need to resize the file first.
There seems to exist program provided by TI that allow users to transfer texts, pictures and other documents to your calculator devices. You can also possibly find small utilities on fan Nspire sites that offer various forms of conversions for transferring of files to calculators.Open TIFF file Open TNS file