Interesting web app. I do the same thing manually — start with an image I like (i take a lot of images just for the color combo) and extract portions to create a palette. It works! I uploaded one of my old images…
http://wellstyled.com/tools/colorscheme2/index-en.html
http://www.easyrgb.com/harmonies.php
Apps:
ColorDesigner
Color Clucker
ColorWrite
I use the old (free) version of ColorDesigner but may consider giving 10 bucks for the new one.
The free ColorWrite is awkward but does include a CMY subtractive scheme to let you dabble with rather than the additive RGB.