Invert colors of a ScreenShot

HI,

for documentation purpose I do ScreenShots with Spectacle. I want to
invert the colours of the ScreenShots and use 'convert <infile> -channel
RGB -negate <outfile>' at the CLI afterwards.

I think this could be simpler ...
How to create a entry in a context menu (e.g. from Dolphin) to call a
little bash-Script to do this?

Bernd