This is a Heroku buildpack for vendoring the ImageMagick binaries into your project.
This one actually works :)
In your project root:
heroku buildpacks:add https://github.com/mkmms/heroku-buildpack-imagemagick --index 1 --app HEROKU_APP_NAME
"index 1" means that imagemagick will be installed first.
Go to https://www.imagemagick.org/download/releases and find a version you want (*.tar.gz). Edit the
bin/compile file and change out the version number. Clear cache, as shown below, and redeploy your app to Heroku.
Since the installation is cached you might want to clean it out due to config changes.
heroku plugins:install heroku-repo
heroku repo:purge_cache -app HEROKU_APP_NAME
Copy the snippet above into CLI.