1 - Where are the album art pictures stored?
On Windows they are stored in %appdata%\vlc\art (enter into Windows Explorer address bar or "Start >> Run..."). On WinXP that is the same as:
C:\Documents and Settings\(Your username)\Application Data\vlc\art
2 - In one of my albums, a picture completely different from the correct one was loaded. How can I indicate that this match is wrong, so that it does not happen again?
IIRC VLC uses a Google Image search query with the track title/metadata and just uses the first result (so not customisable).
EDIT: Actually the Google Image search script is a Lua script which you could edit. It is in VLC_ROOT_DIR\lua\meta\ (where VLC_ROOT_DIR is the location of VLC's files).
END OF EDIT.
Arite.