If you upload a TIFF file, then downloading the original will give you the file in the exact format you uploaded it with - with whatever compression was applied prior to uploading still intact.

If you upload another image format - e.g. a JPG, and then ask for this to be converted to a TIFF at point of download (e.g. via a conversion pre-set), then Asset Bank will convert the original file to a TIFF with LZW compression applied to it and deliver this to the users desktop.