Subject of the issue
It would be great to add ImageMagick installation instructions because this package is not working with default ImageMagick 7 installation.

They removed convert (now you should use "magick" command) and you have to select this checkbox to add convert:

Info from http://imagemagick.org/script/porting.php#cli
"The "magick" command is the new primary command of the Shell API, replacing the old "convert" command. "
Your environment
- Operating system
- Node.js version (
node --version)
- ImageMagick version (
identify -version)
- s3-uploader version