Can I actually design postcards with these applications? Can they do CMYK conversions?
I think that you can probably design postcards with these products but I doubt that any of these applications can output CMYK files. LiveQuartz is free and GraphicConverter works as a full-feature demo so you might want to try. I haven't seen if PixelMator supports CMYK conversion.
You could also try to download Caffeine Software applications (
here). The company is dead but its software is still available for download. TIFFAny might offer what you need but, as far as I remember, the interface is weird and non-intuitive and I never had the nerves to try for more than a couple of minutes.