You will need one of the versions that includes DLL in the filename. Choose 64bit if you are sure that the O/S will always be 64bit, otehrwise just use the x86 version. Ensure you restart after installing ImageMagick.
Note: ImageMagick is licensed under Apache, which is not overly restrictive (View details)
The newer version of ImageMagick also supports Ghostscript to add support for loading of PDF and PS files (as rasterized images), i.e. you must install ImageMagick and Ghostscript to support PDF/PS.
Remember when installing ImageMagick to download the version that matches the bitness of the application you are compiling. I.e. if you are creating a 32bit application in Delphi then ensure that you install the 32bit verison of ImageMagick.