Yeeouch, tried to install ImageMagick on one of the Mediawiki wikis this afternoon and it didn't want to play. There are several attempts at instructions from different people, each with some slight variation and none worked for us. If I, or one of my colleagues, gets it working I'll post here how we did it. If someone is reading this and has done it please let me know how.
My environment is IIS 6, MediaWiki 5.6, PHP5, MySQL 5, ImageMagick 22.214.171.124.
Update: You don't need ImageMagick, just enable GD – see comments below.