wkhtmltoimage binary is not included in this buildpack.
This buildpack only installs wkhtmltopdf, it isn't very useful by itself. You'll probably want to use add it to you current buildpacks config.
$ heroku buildpacks:add https://github.com/notvad/wkhtmltopdf-buildpack
Remember to clean your repository cache if you are updating the version of buildpack. To do that, run:
$ heroku plugins:install https://github.com/heroku/heroku-repo.git $ heroku repo:purge_cache -a appname
Copy the snippet above into CLI.