Installing Imagemagick using Homebrew does not give me the imagemagick command
There is no imagemagick command. /usr/local/Cellar/imagemagick/ is the Homebrew folder containing the content of the package. To see the names of the installed commands run

$ ls /usr/local/Cellar/imagemagick/*/bin
Magick++-config     compare         display         magick-script
MagickCore-config   composite       identify        mogrify
MagickWand-config   conjure         import          montage
animate             convert         magick          stream