I've installed MusicBrainz Picard to organize my music collection, mainly to add covers and album titles to my tracks. However, it doesn't work perfectly, as in some cases no information is added although I can see the track/album cover in Google. How can I add other search engines, e.g. Amazon or Google, to make the process more accurate?
Picard really isn't the best tool to use for cover art. Picard was designed mainly as a GUI for MusicBrainz's metadata database and cover art was added as an afterthought.
IMHO, the best tool for finding album art is Album Art Downloader (http://www.hydrogenaud.io/forums/index.php?showtopic=57392) (developed by one of our own forum members) -- which searches multiple sites (including Google Images and Amazon).
If that seems overkill for you, in my experience the best one-stop source I've found is Album Art Exchange (http://www.albumartexchange.com/). (Registration required, but it's basically a forum account and you won't get spammed.)
Coverlovin (https://launchpad.net/coverlovin) is a command-line tool written in python. I don't know how cross-platform that is or if it can be made to work in Windows, but it seems fine in Linux. It is recursive, and works its way through a directory tree effortlessly.
I let it loose on my low[-er] bitrate for portable listening library because I no longer know enough shell to recursively find and copy to the appropriate place in another tree.
(by the way, pacpl is my fave recursive audio converter tool, but no, it does not know about album art, only audio files --- and I don't know any python to attempt to add to it)