I was thinking of using Imagemagick's convert program, as my preferred program nconvert doesn't work on the ARM CPU of the Raspberry pi.
Looking at the convert
documentation , you can use a tiff:-
to represent a tiff file on the standard input or output stream. And scanimage docs say you need to give the format to get TIFF. Putting this together, you'd have:
scanimage --format=tiff | convert tiff:- scan.jpg
I don't have access to a machine where I can test this.