Protect images on website from piracy through signature/invisible watermark

Protect images on website from piracy through signature/invisible watermark

Introduction
Many websites provide lots of images which should be protected from
piracy, eg online shops, artists portfolios, etc.
The first step against this evil is to add a nice visible watermark. Easy.
Though, anyone can easily remove this from the image. So, the second step
is to vary the watermark itself a little (eg rotate, scale, move by a
small amount or distort a bit). This makes life harder for the attacker,
but with a little patience he will remove the watermark, too.
Eventually, one can make every visible watermark invisible. Therefore,
invisible watermarks (or image signatures) were invented. There is lots of
research available on this topic. From simple approaches which alter the
LSB of each pixel to very advanced block-based techniques involving the
wavelet transform.
Question
I'm running Ruby on Rails on Heroku (so some command-line tools are
available, eg Imagemagick). Are there ready-to-use solutions (preferably
open source) for invisible watermarking/image signatures available?
Also interesting:
webservices
C/C++ libraries
non-Rails/Ruby/Heroku solutions