It looks like that png to tns conversion represent attempts in transferring picture files (.jpg, .gif, .png etc.) to TI-Nspire series of graphing calculators developed by Texas Instruments. Possibly only certain pixel sizes of the pictures can be transferred, so you may need to resize the file first.
You can find a special utility from TI which 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 PNG file Open TNS file