I can send and receive files on the Portfolio via xterm2, on my modern PC I use TeraTerm to communicate with xterm2 over serial using the XModem protocol.
I also wrote an image converter to convert jpg to pgf (portfolio graphics file). I haven't yet implemented the compression to write pgc (portfolio graphics compressed) files directly. I use pgcompress on the atari to convert pgf to pgc.
Pgc files can be viewed on the Atari using pgshow.